Can you play a reaction spell like Wind Wall without a legal target in order to trigger 'when you play a spell' effects?

Ruling: You cannot play a reaction spell like Wind Wall without a legal target, even if you want to trigger other effects that care about playing spells.

Nuances:

• This applies even when you would benefit from playing the spell for secondary effects (such as boosting a unit that gains +1 when you play a spell)