Update slot.go

Compatible with old version migrating_slot  is -1 to return null

Author: hll1213181368

store/slot.go

@@ -227,13 +227,7 @@ func (s *MigratingSlot) UnmarshalJSON(data []byte) error {
 
 func (s *MigratingSlot) MarshalJSON() ([]byte, error) {
 	if !s.IsMigrating {
-		// backwards compatibility. When we read from an old cluster that had `-1`
-		// denoting !isMigrating. The MigratingSlot field will not be nil. So when
-		// this field is marshal'd back into JSON format, we can keep it as it was
-		// which was "-1".
-		// The only case this turns back to null is if a migration happens on this
-		// shard, and the function `ClearMigrateState()` is called on the shard.
-		return json.Marshal(NotMigratingString)
+		return json.Marshal(nil)
 	}
 	return json.Marshal(s.String())
 }