def _correct_folder(folder: str) -> str:
  """Ensures the folder follows a standard.

  Pathlib.parent in the root folder results in '.', whereas in other places
  we should use '' for the root folder. This function makes sure the root
  folder is always empty string.

  Args:
    folder: the folder to be corrected.

  Returns:
    The corrected folder.
  """
  if folder == '.':
    return ''
  return folder