Can a spell that targets something be played if there are no valid targets?

Ruling: No, you cannot play a spell that targets if there are no legal targets available. You must have legal targets to put a spell on the chain and finalize it.

Nuances:

• Spells that say "up to ___ targets" or "any number of targets" can choose to target zero units
• Falling Star is a special case because the spell itself doesn't target (the reflexive triggers it generates do target), so you can play it without a legal target and it just won't generate its triggers