Priority and timing between enchantments and instants
It's not "immediately after" at all. Between the bolt resolving and the strength resolving, the game goes through a whole bunch of steps that are usually glossed over in casual play.


State-based actions are checked and then performed. This includes, but is not limited to, destroying creatures with lethal damage dealt to them, and causing players with 0 or less life to lose.
Repeat 1 until nothing new happens.
Abilities that triggered during the resolution of the bolt (e.g. "Whenever this creature is dealt damage") and during steps 1 and 2 are put on the stack.
Repeat 2 and 3 until nothing new happens
Players gain priority and may play instants and spells with flash.
If no player played anything, now the Unholy Strength resolves, sees that it has no valid target, and is removed from the stack to your graveyard with no effect.