Core Hook - Remove temporary fix and document possible null in hook (#805)

* Remove workaround

Partially revert temporary crash fix https://github.com/azerothcore/azerothcore-wotlk/commit/a36e94a49a5b3bbdf01c622600c87da8cf55a26b

* Update ScriptMgr.h

src/server/game/Scripting/ScriptMgr.h

@@ -445,6 +445,7 @@ public:
     virtual void OnDamage(Unit* /*attacker*/, Unit* /*victim*/, uint32& /*damage*/) { }
 
     // Called when DoT's Tick Damage is being Dealt
+    // Attacker can be NULL if he is despawned while the aura still exists on target
     virtual void ModifyPeriodicDamageAurasTick(Unit* /*target*/, Unit* /*attacker*/, uint32& /*damage*/) { }
 
     // Called when Melee Damage is being Dealt