Yes, that's grammatical (as would be "nihilum supernum"). Those are closer to English "nothing" than "nothingness", and maybe too short to fit with the preceding lines, but I don't know if that's an issue.
This is a monolingual dictionary of medieval Latin, and the uses of "nihilitas" it quotes have a distinct moral connotation of humility/self-abjection (kind of like the English "I am nothing before you"); I can't find other uses of the term either. So I would probably steer away from 'nihilitas'.
Depending on how 'physical' that "nothingness" is supposed to be, I would go for either just "nihilum" (more abstract) or "inanitas"/"vacuitas" (more concrete), as in the translation of Genesis "et te...
If anyone can do non-wrong Latin, I could use a translation of the following for HPMOR. The original is supposed to be circa 1200.
No rescuer hath the rescuer.
No Lord hath the champion,
no mother and no father,
only nothingness above.