He can't be a Nobody. Not because he has a Heartless and Nobody of his own, but simply because Nobodies didn't exist back then.
This is all conjecture, but he COULD, however, have no heart (ie No Heart) as, say, a being originally created without a heart, unlike Nobodies which once had hearts. How would he create a Heartless and a Nobody? Taking the heart from someone else or creating one from nothing. Xemnas is capable of taking the heart from a living being, and Xehanort's original goal in the Ansem Reports, if you read up on it, is to make a heart from nothing.