message: do not show error when value is null
  null value should show empty input with no error. It's used for showing empty input or in case of clearing action
diff: @@ -99,8 +99,13 @@ class DateTextField extends PureComponent {
  } = this.props;
  if (!value.isValid()) {
  + // if null - do not show error
  + if (value.parsingFlags().nullInput) {
  + return '';
  + } else {
  return invalidDateMessage;
  }
  + }
  if (
  (maxDate && value.isAfter(maxDate)) ||
  