def reduce_dataset_name(key):
    """Divide a dataset name into is base and modifier

    Args:
        dataset_name (str): Key to reference a dataset that may or may not have
            a modifier suffix
    Returns:
        tuple of (str, str or None): First string is the base key, the second
        string is the modifier.
    """
    if key.endswith('/missing'):
        return tuple(key.rsplit('/', 1))
    return key, None