Can I cast a spell without legal targets (like Stupefy) just to draw a card?

Ruling: No, if a spell needs a target, you can't cast it without a valid target.

Nuances:

• If there is a valid target when you cast the spell, but the target becomes invalid before resolution, you'll still draw the card
• You cannot cast a spell with mandatory Deflect cost and decline to pay it (you can only decline Deflect for triggered abilities)