Thus a zero BTC transaction even w/ a fee could be pruned. When the fee gets transfered to another address and that transaction is deep enough the 0 BTC transaction is eligible for pruning.
0-value outputs
can be spent (uselessly), so these outputs can't be pruned. The fee never has anything to do with pruning.
So as long as the address is to "nowhere" (no known private key) then the transaction can't be pruned because there will never be a subsequent transaction using that output as an input.