code: @SuppressWarnings("unchecked")
      public static <D, R> Mapping<D, R> nullMapping() {
          return (Mapping<D, R>) NULL_MAPPING;
      }
docs: Retrieves the <code>null</code> mapping, which maps each domain value to <code>null</code>.
  
  @param <D>
  domain class.
  @param <R>
  range class.
  
  @return the <code>null</code> mapping.