Core/Spells: Always add threat for spells from spell_threat table, not only if target was hit. Unsuccessful cast results in 0 threat added instead of the full amount and creating threat list entry for caster. This fixes various exploits with taunt spells bugging out scripts due to taunt immunity/passive state (creatures would immediately evade resetting script state if no entries were present on threat list - entry with no threat is not the same as no entry)

Closes #11883