What happens if I choose the second option, but do not have an action card in hand? (This is not the same as the accountability issue mentioned earlier -- I can have zero cards in hand, or I can even reveal them though I am not forced to do so.) The FAQ specifically says I can choose an option even if I know I can't do it, so the answer isn't "you can't choose that option".
The relevant text of the card is "...or trash an Action card from your hand and gain a card costing up to 3 Coins more than it."
There are a couple of different interpretations that make sense to me:
1. "trash a card" and "gain a card" are not separate clauses. You either do everything in the single clause "trash a card then gain a card" or you do none of it. So if you don't trash an action card, you gain nothing. This is supported by the FAQ on Governor, which has the same wording, and so I'm pretty sure this is the right interpretation, but the FAQ on Graverobber doesn't mention it so I thought I'd check.
2. They are separate clauses. You didn't trash an Action card, so there is no cost. No cost is equivalent to $0. Therefore you may gain a card costing up to $3. This was my initial interpretation, since the text is not the same as the text on Remodel/Expand/Upgrade/Remake. On the other hand, maybe the text is different only because there's too much text on the card, which is supported by the text on Governor. On a third, orthogonal (ewww) hand, is an undefined cost really the same as zero?
3. They are separate clauses. You didn't trash an Action card, so the cost is undefined. Uh...I don't even know where to go from here (do I get to make up the cost since it's undefined and I'm the active player? That can't possibly be right) so I'll just make a programming joke: null pointer exception, the game ends in a draw and you pick a new kingdom.