Core/Creatures: Fix spell_school_immune_mask being applied against positive spells too (#23999)

src/server/game/Entities/Unit/Unit.cpp

@@ -7373,7 +7373,9 @@ bool Unit::IsImmunedToSpell(SpellInfo const* spellInfo, WorldObject const* caste
                 continue;
 
             SpellInfo const* immuneSpellInfo = sSpellMgr->GetSpellInfo(itr->second);
-            if (!(immuneSpellInfo && immuneSpellInfo->IsPositive() && spellInfo->IsPositive() && caster && IsFriendlyTo(caster)))
+            // Consider the school immune if any of these conditions are not satisfied.
+            // In case of no immuneSpellInfo, ignore that condition and check only the other conditions
+            if ((immuneSpellInfo && !immuneSpellInfo->IsPositive()) || !spellInfo->IsPositive() || !caster || !IsFriendlyTo(caster))
                 if (!spellInfo->CanPierceImmuneAura(immuneSpellInfo))
                     schoolImmunityMask |= itr->first;
         }