Rotting Regisaur

• WWizards of the Coast2019-07-12
  Official Ruling
  If you have no cards in hand when Rotting Regisaur’s triggered ability resolves, you simply don’t discard any cards. There’s no penalty for being unable to do so.
• WWizards of the Coast2019-07-12
  Official Ruling
  Because the upkeep step is before the draw step, you discard for Rotting Regisaur’s triggered ability before you draw a card for your turn.

Rulings via Scryfall