Gandalf, Spark Starter

You choose how many targets Gandalf's triggered ability has and how the damage is divided as you put the ability on the stack. Each target must receive at least 1 damage.

If some of the targets are illegal targets as Gandalf's triggered ability tries to resolve, the original division of damage still applies but no damage is dealt to the illegal targets. If all targets are illegal, Gandalf's triggered ability doesn't resolve.