def _parse_none(arg, fn=None):
    """Parse arguments with support for conversion to None.
    
    Args:
        arg (str): Argument to potentially convert.
        fn (func): Function to apply to the argument if not converted to None.

    Returns:
        Any: Arguments that are "none" or "0" are converted to None;
        otherwise, returns the original value.

    """
    if arg.lower() in ("none", "0"):
        return None
    return arg if fn is None else fn(arg)