def null_distance_results(string1, string2, max_distance):
    """Determines the proper return value of an edit distance function
    when one or both strings are null.

    **Args**:

    * string_1 (str): Base string.
    * string_2 (str): The string to compare.
    * max_distance (int): The maximum distance allowed.

    **Returns**:
    -1 if the distance is greater than the max_distance, 0 if the\
        strings are equivalent (both are None), otherwise a positive number\
        whose magnitude is the length of the string which is not None.
    """
    if string1 is None:
        if string2 is None:
            return 0
        else:
            return len(string2) if len(string2) <= max_distance else -1
    return len(string1) if len(string1) <= max_distance else -1