Leech Collector // Bloodletting

If a prepare spell with one or more targets has no legal targets when it tries to resolve, it won't resolve and none of its effects will happen. Since the spell was cast, the associated permanent will still not be prepared.

A preparation card is a creature card in every zone. For example, while it's in your graveyard, Emeritus of Ideation is a blue creature whose mana value is 5. It can't be the target of Zealous Lorecaster's triggered ability ("When this creature enters, return target instant or sorcery card from your graveyard to your hand.").

Being prepared isn't a copiable value. If a permanent becomes a copy of a prepared creature, it won't be prepared. That permanent will have the alternative characteristics, though, and some other effect could cause it to become prepared later.