Core/Spells: Fix Vanish's stealth trigger and Demonic Empowerment's invisibility trigger

Those spells were failing to pass default effect trigger spell due to their custom implementation (changing triggered_spell_id instead of adding serverside spells with proper targeting).

src/server/game/Spells/SpellEffects.cpp

@@ -1593,8 +1593,8 @@ void Spell::EffectTriggerSpell(SpellEffIndex effIndex)
                 if (unitTarget->ToPlayer()->HasSpellCooldown(1784))
                     unitTarget->ToPlayer()->RemoveSpellCooldown(1784);
 
-                triggered_spell_id = 1784;
-                break;
+                unitTarget->CastSpell(unitTarget, 1784, true);
+                return;
             }
             // Demonic Empowerment -- succubus
             case 54437:
@@ -1604,8 +1604,8 @@ void Spell::EffectTriggerSpell(SpellEffIndex effIndex)
                 unitTarget->RemoveAurasByType(SPELL_AURA_MOD_STUN);
 
                 // Cast Lesser Invisibility
-                triggered_spell_id = 7870;
-                break;
+                unitTarget->CastSpell(unitTarget, 7870, true);
+                return;
             }
             // just skip
             case 23770:                                         // Sayge's Dark Fortune of *