code: public static void removeNullsAndRelations( Map<String, Object> entityMap )
    {
      Iterator<Map.Entry<String, Object>> entryIterator = entityMap.entrySet().iterator();
      while( entryIterator.hasNext() )
      {
        Map.Entry<String, Object> entry = entryIterator.next();
        if( !isSystemField( entry ) )
        {
          if( isNullField( entry ) || isRelationField( entry ) )
          {
            entryIterator.remove();
          }
        }
      }
    }
docs: Removes relations and {@code null} fields (except system ones) from {@code entityMap}.
  System fields like "created", "updated", "__meta" etc. are not removed if {@code null}.
  
  @param entityMap entity object to clean up