Can you cast Defy with no valid targets?

Ruling: No, you cannot cast Defy with no valid targets. The spell fails the legality check before it can be added to the chain.

Nuances:

• You can Defy your own spell, but only the Defy itself would count as a played spell (the defied spell doesn't count as being played)
• If you Defy your own spell to buff something like Ravenbloom Student, you only get +1 from Defy resolving, not from the countered spell
• Attempting to cast Defy on an untargetable spell (like Falling Star) would fail the legality check and you'd reveal your card for nothing