def dictlist_convert_to_string(dict_list: Iterable[Dict], key: str) -> None:
    """
    Process an iterable of dictionaries. For each dictionary ``d``, convert
    (in place) ``d[key]`` to a string form, ``str(d[key])``. If the result is a
    blank string, convert it to ``None``.
    """
    for d in dict_list:
        d[key] = str(d[key])
        if d[key] == "":
            d[key] = None