code: public static <T> Mapping<T> defaultv(Mapping<T> base, T defaultVal, Constraint... constraints) {
          return optional(base, constraints).map(o -> o.orElse(defaultVal));
      }
docs: (mapping) return default value, if input value is null or empty
  NOTE: all options operations will be delegate to 'base' mapping
  @param base base mapping
  @param defaultVal default value to be used when the related input is empty
  @param constraints constraints
  @param <T> base type
  @return new created mapping