def to_list(items):
    """
    In many places we accept either a single string or a list of strings. This function
    normalizes None -> [None], str -> [str], and leaves lists unchanged.
    
    Parameters
    ----------
    items : str, list, or None

    Returns
    -------
    list
    
    """
    if not isinstance(items, list):
        items = [items]
    return items
