fix(DB/Spells): define Dust Field as a positive spell (#7798)

- Closes #5392
- Closes #6285

data/sql/updates/pending_db_world/rev_1631201007246764700.sql

@@ -0,0 +1,5 @@
+INSERT INTO `version_db_world` (`sql_rev`) VALUES ('1631201007246764700');
+
+DELETE FROM `spell_custom_attr` WHERE `spell_id`=21909;
+INSERT INTO `spell_custom_attr` VALUES
+(21909,0x06000000);

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

@@ -12119,7 +12119,7 @@ bool Unit::IsImmunedToSpell(SpellInfo const* spellInfo)
         {
             SpellInfo const* immuneSpellInfo = sSpellMgr->GetSpellInfo(itr->spellId);
             if (((itr->type & spellInfo->GetSchoolMask()) == spellInfo->GetSchoolMask())
-                    && !(immuneSpellInfo && immuneSpellInfo->IsPositive() && spellInfo->IsPositive())
+                    && !(immuneSpellInfo && immuneSpellInfo->IsPositive()) && !spellInfo->IsPositive()
                     && !spellInfo->CanPierceImmuneAura(immuneSpellInfo))
                 return true;
         }