Oh my God They Banned Kenny
The DQB doesn't say, but I would presume that an empty Bridgehead would also count as an empty hex.
Yes, I would proceed based on the "without a defending ground unit" criteria. I think it just listed airbase, air or naval unit to emphasize that while those are considered "units" for other purposes, their presence does not disqualify the hex from being considered "vacant" for this purpose. I do not believe it was intended to be an exhaustive list of exceptions.