You have to discard, but have no cards in your hand
Nothing happens.



He is instructed to discard a card.


  701.7a To discard a card, move it from its owner’s hand to that player’s graveyard.


It's impossible for him to discard a card, so he does as much as possible.


  609.3. If an effect attempts to do something impossible, it does only as much as possible.


It does not affect him.