If you have at least 1 card in your Graveyard, you can activate “Emperor Sem’s” effect even if your opponent has no cards in their Graveyard.
“Emperor Sem’s” effect starts a Chain. If an effect is chained that shifts control of the Tribute Summoned monster to your opponent, “Emperor Sem’s” effect resolves as normal. However, if control of “Emperor Sem” shifts to the opponent as part of the Chain, the effect of “Emperor Sem” is not applied because it is no longer a case where ‘you Tribute Summoned a monster’, as “you” now refers to your opponent.
The effect targets the cards to be returned to the Deck.
When this card activates its effect, your opponent does not need to apply the effect - he can choose to not return a card to the Deck. When your opponent chooses to return a card, he chooses the target, not you.
If your opponent has no cards in his Graveyard, you can still activate the effect of "Emperor Sem" as long as you have one or more cards in your Graveyard. You cannot activate the effect of "Emperor Sem" if you have no card in your Graveyard.
When a monster is successfully Tribute Summoned or Tribute Set, the player who controls "Emperor Sem" chooses whether or not the effect activates. When you activate the effect of your "Emperor Sem", you must return a card to your Deck, but your opponent may choose to place a card to the top on his Deck or not.