Sorry to say that, but the currently best answer on this seems to be only: "we also have not been able to locate it by searching the net".
To conclude from that that it doesn't exist in the papal archives or other books not digitized –– seems a bit premature?
The question comes in three interrelated but distinct parts:
––# 1. Did Urban issue a bull of that name in 1095? 2. Is the 'bull' origin of that term? 3. Is "terra nullius" a legal concept that is reflected in anything Urban issued at the time when calling for the crusade and offering perks?
- Unless someone digs through printed books or files at Vatican Secret Archives (location known and open to scholars) and concludes after thourough search: "nope, no litteræ apostolicæ bearing the title "Terra nullius". It is best to conclude "perhaps yes, many scholars mention it". And not only did it enter legal discourse of international relations as late as being invented when Westerners going to Australia, but before and at the same time that Urban II used it as well – and that it was also the basis for "how to handle the Americas"
Gaius on the Occupation of terra nullius ("insula nata"):
The hypothetical case of an »island which rises from the sea« (insula nata) served as an example of the principle that territories not in anyone's possession can be occupied by the first-comer who has the intention to establish his sovereignly there.
Inst II, 1,22 (CIC 1，49).
Insula quae in mari nata est, quod ra re accidit, occupantis fit: nullius enim esse cfeditur.
(if an island arise in the sea the sea, an uncommon event, it is open to occupation for it is regarded as belonging to no one.)
–– Wilhelm G. Grewe (ed): "Fontes Historiae Iuris Gentium – Sources
Relating to the History of the Law of Nations, Vol 1, 1380 v.Chr./B.C. – 1493", deGruyter: Berlin, New York, 1995.
It is said that this 1095 papal argument was the predecessor of the argumentation in the bull Inter Caetera from 1493, which is online in English. Land occupied by unbelievers is unoccupied land after all. (Cf Andrea Weindl: "Inter caetera, mare liberum und terra nullius - das europäische Völkerrecht und die außereuropäische Welt", in: Inken Schmidt-Voges et al. (eds), "Pax perpetua. Neuere Forschungen zum Frieden in der Frühen Neuzeit", deGruyter, 2010.)
Looking at an admittedly incomplete list of bulls on Wikipedia that only mentions one of Urban's bulls – not only from the year in question but in total! – just proves that Wikipedia is incomplete.
He issued more letters, and more letters in that year. Not only the listed one about the kingdom of Aragon, but two about cloister and convent affairs. (Also not quoted but numbered in list here.) And in 1091 the bull "Cum omnes insulae" – which happens to grant Corsica to the bishop of Pisa (text). (Found via Las bulas alejandrinas de 1493 y la teoría política del Papado medieval : estudio de la supremacía papal sobre islas, 1091-1493 with a ToC & intro.)
No, the term and legal concept came from Roman law and was used throughout the middle ages. It began to take concrete shape after jurists pondered about newly created land, like "insula inter mari nata", island emerging from the sea.
In Roman law, the main class of objects which fell into the category of terra nullius was ferae naturae, i.e. wild animals and fish. A wild creature belonged to everybody and nobody until such time as a person killed or captured it at which point it became the private property of the killer/captor (even if he was a trespasser) rather than the person on whose land it was found. From this developed the notion of terra nullius which (although it was gradually extended 'to justify acquisition of inhabited territories by occupation if the land was uncultivated or its indigenous inhabitants were not "civilized" or not organized in a society that was united permanently for political action') was initially applied to the acquisition of 'new' uninhabited territory.
–– Jane Morgan: "Digging Deep: Property Rights In Subterranean Space And The Challenge Of Carbon Capture And Storage"
That Urban did offer perks seems evident. That these not only addressed spiritual benefits in the afterlife is also clear. How he argued that material benefits in the secular world fit into or are justified from his Christian bishop's view is the one thing even more relevant than finding a bull of the very title. Since the quotes discussed so far often just declare "Pope said it in 1095" without pointing to a more exact location, it might very well be that he did convey a message that follows suit on the exact legal concept in his interpretation, even if it's buried in one of the documents and that just the "bull-title" part is the result of Chinese whispers effect.
One such paper making the connection between Urban and terrae nullius, that may either omit "bull" or partly source/represent such a confusion, if it is one, might be exemplified by this paragraph:
The representations of non-Christian, non-European people made by Pope Urban II, Vitoria, Locke, Vattel and Dampier provided Cook and Banks with accepted ideas that informed their characterisations of the native inhabitants of Australia. The status of Aborigines was initially constructed on representation of them as primitive barbarians. Without prospects of developing trade relations and the perception that Aborigines posed little military threat, no treaty was concluded between the British Crown and theAborigines. As settlement proceeded on the basis that Australia was a terra nullius, Aboriginal resistance brought conflict, which gave them the status of enemies of the state. As resistance diminished, individual Aborigines were brought into colonial society as violators of colonial law or as wards of the state. Similar to other European colonies, the construction of Aborigines brought with it a perceived responsibility for the Crown to uplift the natives from their apparent primitive state. The authority to uplift Aborigines was clearly articulated by Governor Hindmarsh in the Proclamation of South Australia in 1836.
–– Jackie Delpro: ""THE TIDE OF HISTORY": Australian Native Title Discourse in Global Perspective", MA Thesis, Victory University Sidney, 2003 (PDF)
That the very term "terra nullius" itself would be just a late invention "myth", not even applicable to the age of discovery has been argued.
The “Doctrine of Discovery” and Terra Nullius: A Catholic Response, Concacan, 2016. (PDF)
The challenge, from a historical point of view, is that the term terra nullius is not as old as its Latin name suggests. While the “law of the first taker” existed in Roman Law, it generally applied to things like wild animals. The term terra nullius, however, was not used at all until the late 19th century and was at that time mainly confined to disputes over Antarctica and the North Pole.
This is recurring to
Terra nullius, it seems, was an impostor. Debate is turning to why we embraced this legal fiction. […] If terra nullius was not employed in the eighteenth and early nineteenth centuries to justify dispossession, where did it come from? It is remarkable that even during the ascendancy of terra nullius as an historical tool no historian managed to answer this question and it remains unanswered. Most early and mid twentieth-century sources identify the polar regions debate of the late nineteenth century as the origin of the idea terra nullius…
Here historians have made an effort to explain that terra nulliuswas derived from the Roman law doctrine of res nullius. But these efforts at historical clarification have just added to the layers of ambiguity and confusion, as there was no Roman law doctrine of rex nullius. The relevant passage of Roman law is the law of the first taker, or the law ferae bestiae — literally, the law of wild beasts — in which the word nullius, 'nobody's', was employed. Ferae bestiae states that any thing, such as a wild beast, that has not been taken by anybody becomes the property of the first taker. The Roman law Institutes of Justinian provided the longest discussion and made the connection between the law of the first taker and natural law:
Now things become the property of individuals in many ways: for of some things ownership arises by natural law which, as we have said is called the law of nations [iusgentium], and of others at civil law. It is more convenient to start with the older law and, obviously, the older law is natural law which the nature of things introduced with humankind itself
… Hence, wild animals, birds and fish, i.e. all animals born on land or in the sea or air, as soon as they are caught by anyone, forthwith fall into his ownership by the law of nations [iusgentium]: for what previously belonged to no one is, by natural reason, accorded to its captor [quod enim ante nullius est id naturali ratione occupanti conceditur].
–– Andrew Fitzmaurice: "The genealogy of Terra Nullius", Australian Historical Studies, 38:129, 1-15, DOI
Sounds nice to find an historical invention or myth?
That cannot be true either.
As for example Samuel Pufendorf wrote in De habitu religionis christianae ad vitam civilem,, 1678, that the land taking of the Israelites after the Exodus was justified, as the land of Canaan was:
Sic Moses cum populares suos in Aegypto in liberum populum erigere non posset, in deserta loca, et ab humano imperio vacua eosdem eduxit, quoad terram Canaan deletis veteribus incolis occuparent. Huc antequam delati forent Israëlitae, nihilominus liber, suique juris populus fuere, nullius alterius imperio obnoxius, et qui ex loco, ubi peregrinabatur, temporarium imperium haut subierat, partim quod illae terrae nullius essent, partim quod per alios fines ad modum exercitus transiret, cui libertas sua gladio asseritur, et imperia domini territorii armis eluduntur.
(horrible English translation)
The New World, on paper, was legally "vacant" -- terra nullius or vacuum domicilium in Latin. Title to all Indian land is thus held by the discoverer, and Indian people are subject to the overriding political sovereignty of the discoverer! How was this justified? […] The answer is because the land of Canaan was inhabited.
–– Steven Paul McSloy: "Because The Bible Tells Me So": Manifest Destiny and American Indians", St. Thomas Law Review vol. 9, no. 1, Fall 1996, pp. 37-48.
A more subtle retracing of the evolution of the relationship between legal concept and precise term, although equally only on firm footing after Columbus, in: –– Yogi Hale Hendlin: "From Terra Nullius to Terra Communis: Reconsidering Wild Land in an Era of Conservation and Indigenous Rights", Environmental Philosophy 11:2 pp. 141–174
But since all short-tempered SE readers are already
anxious to divide up the plunder eager to read a result:
It is at least very plausible that Urban used the old concept to give it a new interpretation at the time. For the alleged bull with that very title, we netizens are hampered by what we find for this online: in this case often something between hearsay and recursive indirect citations. That is far from good. But also far from enough to call "Urban wrote (about) that" "a myth". Such a verdict is possible, but it is not the last word on it now either.
A papal bull, letter, decree with that exact name seems not very likely to exist, at this time of answering. It looks like that bit is the result of Chinese whispers conflation. That Urban used the precise term is difficult to ascertain or refute absent access to the sources. However, that Urban might have used the concept and the meaning of "terra nullius" in writing or at least when preaching sermons seems very possible.
Confer –– Alfons Becker: "Papst Urban II (1088-1099): Der Papst, die griechische Christenheit und der Kreuzzug", Schriften der Monumenta Germaniae Historica Volume 19, Hiersemann, Stuttgart, 1988. (Detailing from the Canon of Clermont the distinction between motivation (pure spirituality) and secular goals (permanent Christian land ownership), p384.)
What Urban did write is relatively easy to follow in a more comprehensive manner than Wikipedia thinks relevant in a Latin collection of his works:
–– Urban, Pope; Mathilde, of Swabia; Eugène de Rozière; J -P Migne: "B. Urbani II pontificis Romani Epistolae, diplomata, sermones", Patrologiae cursus completus. Series latina, t. 151, Turnholti, Belgium : Typ. Brepols, [1978?] 1853.
Sadly, a cursory reading did not come up with "terra nullius".
But somewhere seems to exist a document, from Urban, from 1095, called "Inter Fines Expeditionis Sacrae Crucis Liberationem Ecclesiarum Orientalis". (pointer)