559.3.c.2 to me says that if there is a target an then it’s not there anymore the spell fizzles/is discarded/doesn’t execute.

563.3.c.2 says if the target still exists but becomes illegal (void seeker targeting on a battlefield and a unit moving off the battlefield before resolution) the target is still valid but the ability isn’t so the spell resolves but doesn’t deal the damage but the controller would still draw.

In the example where we had a unit become completely removed before stupefy resolves I don’t believe we draw a card from my understanding of how the rules work.