code: public static AllowableValues emptyToNull(AllowableValues newValue, AllowableValues current) {
      if (newValue != null) {
        if (newValue instanceof AllowableListValues) {
          return defaultIfAbsent(emptyListValuesToNull((AllowableListValues) newValue), current);
        } else {
          return defaultIfAbsent(newValue, current);
        }
      }
      return current;
    }
docs: Retains current allowable values if then new value is null
  @param newValue - new value
  @param current - existing values
  @return the appropriate value