def add_url_parameters(url, **parameters):
    """ Add url parameters to the base url.

        :param url: base url.
        :type url: str


        :kwargs: key-value pairs of the url parameters to add. If the value is None, the pair is not added.
        
        :return: url string.
        :rtype: str
    """
    parameters = {k: v for k, v in parameters.items() if v is not None}
    if len(parameters) == 0:
        return url
    parameters = "&".join([f"{k}={v}" for k, v in parameters.items()])
    return f"{url}?{parameters}"