When a heartless without a nobody dies, its heart flutters off into a different realm where its body and soul are waiting. If a heartless with a nobody dies, its heart lies in a state of suspension, waiting for its nobody to be defeated so its body and soul can join it.
Basically, the body and soul of a nobody doesn't simply vanish from existence, but it vanishes to wherever its heart is (or if its heart is still a heartless, the nobody gets consumed by darkness).
It's in Another Report, part 8.
Kingdom Hearts Another Report - Secret Report XIII Translation - KH-Vids.Net Forum
These bodies, however, still have hearts to give them existence.