The wording should be "gain 15hp and then destroy a permanent". That is a wording error, not a bug.
The AI will not cast its ability without any permanents to destroy, but players can and will do this if they are up against a permanentless deck and they still want ot benefit from the death effect or from a black hole.
SoFo is not OP because of that, it is OP because it costs 7
(extremely cheap for rainbow decks, very expensive for other decks), can destroy 3 permanents over the course of 3 turns, is virtually unstoppable after gaining 15hp, and it gives the user a free black hole, essentially packing 3 deflags and a black hole into one card.