if `right` might be None you could:

    def combine_path(left, right):
        """
        os.path.join(a, b) doesn't like it when b is None
        """
        right = right or ''
        return os.path.join(left, right)

My main concern here is better variable naming, the above example just shows a common pattern for arguments that can fallback nicely and does not need to be implemented here :)