Core/Spells: mark Arcane Missiles aura as negative. Was downranking the spell when targetting low level mobs due to effect 1 flagged as possitive.

Closes #12152

src/server/game/Spells/SpellInfo.cpp

@@ -2788,6 +2788,9 @@ bool SpellInfo::_IsPositiveEffect(uint8 effIndex, bool deep) const
             // Impact
             if (SpellIconID == 45)
                 return true;
+            // Arcane Missiles
+            if (SpellFamilyFlags[0] == 0x00000800)
+                return false;
             break;
         case SPELLFAMILY_PRIEST:
             switch (Id)