I should roll back this check if it breaks some test cases. But without it, the following schemas are equals, which is not good as well:
A:
   optional group c1 (MAP_KEY_VALUE) {
     repeated group map {
       required binary key;
       optional binary value;
   }
 }

B:
optional group c1 (MAP) {
   repeated group map (MAP_KEY_VALUE) {
      required binary key;
      optional binary value;
  }
}