def join_nullable(l: List[str], sep: str) -> str:
    """
    Filter out None in a list and join the remaining strings.
    Example:
        Input: l=['a', None, 'c'], sep=' '
        Output: 'a c'
    :param l: list of strings
    :param sep: separator
    """
    return sep.join(filter(lambda i: bool(i), l))