Core: define CREATURE_FAMILY_NONE to avoid magic numbers in code

src/server/scripts/Spells/spell_warlock.cpp

@@ -508,6 +508,8 @@ class spell_warl_demonic_empowerment : public SpellScriptLoader
                             case CREATURE_FAMILY_IMP:
                                 targetCreature->CastSpell(targetCreature, SPELL_WARLOCK_DEMONIC_EMPOWERMENT_IMP, true);
                                 break;
+                            default:
+                                break;
                         }
                     }
                 }