We declined this item because this behavior is by design - a null value does not fall within a numeric range. For users that want to allow a null value to pass validation, this can be achieved by inheriting the NumericalRangeValidatorAttribute class and overriding the ValidateCore method.

protected override bool ValidateCore(object value)
{
  if (value == null)
  {
    return true;
  }

  return base.ValidateCore(value);
}