def _unit_conll_map(value, empty):
    """
    Map a unit value to its CoNLL-U format equivalent.

    Args:
        value: The value to convert to its CoNLL-U format.
        empty: The empty representation for a unit in CoNLL-U.

    Returns:
        empty if value is None and value otherwise.
    """
    return empty if value is None else value
