target player discards a card ruling magic

[Evolve]

what happens if target player doesnt have card in hand to discard but spell tells him that he needs to discard a card?

[Twanos]

the discard effect resolves and the players discards 0.

[Arix]

As long as the target is still legal, the spell will resolve and do as much as possible. If you Blightning someone with no cards in their hand, they can't discard, but they'll still take three damage.