Scripts/AuraScript: Make AuraApplication in OnEffectPeriodic never be NULL.

Scripts: remove some unnecessary NULL checks from scripts.

--HG--
branch : trunk

src/server/game/Spells/SpellScript.h

@@ -444,7 +444,7 @@ class AuraScript : public _SpellScript
         Unit* GetCaster() const;
         // returns object on which aura was casted, target for non-area auras, area aura source for area auras
         WorldObject * GetOwner() const;
-        // returns owner if it's unit, NULL otherwise
+        // returns owner if it's unit or unit derived object, NULL otherwise (only for persistent area auras NULL is returned)
         Unit * GetUnitOwner() const;
         // returns owner if it's dynobj, NULL otherwise
         DynamicObject * GetDynobjOwner() const;