I don’t think there’s a clash. Both are cases of the general rule saying ‘Do as much as you can’.
In the Lizzo and Stomper cases, you can always target a unit because you just played one (and there’s nothing in the game which makes them untargetable to your own abilities).
A closer comparison to SotP is Troq’s ult Final Smash: if there’s a tech 0 unit in play, you have to bounce it (even if it’s yours and you wouldn’t want to), but if there are no tech 0 units, you can still cast the spell and get the other effects. You could even play it if there are no units in play at all, and have it do nothing, if for some reason you really wanted to cast a spell and use up 6 gold.
The general ‘Do as much as you can’ rule is the reason you can do these things, not the tiny benefit you could get from playing SotP without a unit to attach it to.