The sentence given by DL speaks about a specific set of hats that have not been found (therefore the definite article in both English and German), your sentence translates as He did not find any hats.

Unless I misunderstand what's going on in this sentence, there's nothing dative here. "Der Hüte" is genitive. It's not changing because of "none" (the "positive"/"negative" distinction you made), but rather because of "of". If, as was asked about elsewhere in this thread, the sentence was "He has found no hats" instead of "none of the hats", you'd use the accusative ("keine Hüte").