Validates whether all required fields of an object have a value:
    - if value empty adds MissingValue errors (if problems accumulator is
      provided)

def ValidateRequiredFieldsAreNotEmpty(gtfs_object, required_field_names,
                                      problems=None):
  """
  Validates whether all required fields of an object have a value:
    - if value empty adds MissingValue errors (if problems accumulator is
      provided)
  """
  no_missing_value = True
  for name in required_field_names:
    if IsEmpty(getattr(gtfs_object, name, None)):
      if problems:
        problems.MissingValue(name)
      no_missing_value = False
  return no_missing_value