Can you activate cards like Defy or Salvage without a valid target or for no reason?

Ruling: You cannot play a spell or activate an ability without a legal target. Defy is not a "may" ability, so if you play it, you must counter a spell.

Nuances:

• You can perform actions on legal targets that result in nothing (redundant actions), such as using a buff effect on an already buffed unit or a ready effect on an already ready unit
• There are scenarios where you might want to play spells not for their own effects but for "when you play..." type triggers (like Darius, Trifarian or Ravenbloom Student)
• A spell cannot target itself (e.g., Defy cannot counter itself)