Yes, I think nullius by itself has not entered the English language, and so shouldn't be added to our word list.
It is part of some Latin phrases used by the legal profession, such as terra nullius, land belonging to nobody, or to no nation, and res nullius, anything belonging to nobody, such as a wild animal. It is also found in the motto of the Royal Society, Nullius in Verba, loosely translated as "Take nobody's word for it".