Call appropriate event for zombies igniting players. Adds BUKKIT-3915
Call PotionSplashEvent even for no effects. Fixes BUKKIT-3618

When a splash potion has no applicable effects we currently do not call
PotionSplashEvent. This means plugins are unable to make custom
potions with reliable splash handling as they have to relicate the
functionality themselves.

With this commit we simply make the event fire regardless of the effects
on the potion.