As a general rule, you can use a discard effect on an opponent who has no cards in hand, it just won't do anything and he won't be considered to have discarded a card.
Ok, thanks for clarification here. It looks like it is a little bit confusing for me to judge if I can use a card, even if it's action could not be triggered, or I can't. Luckily, in most cases using this card would do anything, anyway, so it doesn't matter so much.
In this case I can force opponent to discard, even if he cant' do this. In similar situation, I can cast Chillwind, even if there is no valid target to corrupt?
And I was wondering if I can use Action of Mountain Brigands "Target opponent must give you 1 resource" if opponent has no resources in his pool, but he has means on table to gain resources - by some action, for example. Is Brigands action forcing him to use actions to earn resources, and then give it to me? Or if he has resource tokens on Master Alchemist, Temple of Verena, Sack/Defend Tor Aendris?