what if my hand is empty but i have to discard
what if my hand is empty but i have to discard
You discard nothing... simple as that. Nothing else happens, you just don't discard anything.
Now, if you have to discard a card as part of somethings cost, then you have to have a card to discard, or you're not actually paying the cost.
Additional Info:
The cost of an ability will be on the left of the ":".
Or the card will say something like, "In addition to paying the cost of <this>, discard a card."