def _boolstring(arg):
    """
    Checks api_parser that are not required and have default values on the server. Argument is a string but mus be
    converted to True, False or ''.

    Args:
        arg (str): Parsed argument.

    Returns:
        bool or empty string:
            "True" -> `True`

            "False" -> `False`

            Anything else -> `''`

    """
    if arg == 'True':
        return True
    elif arg == 'False':
        return False
    else:
        return ''