Fix issue when second value of captured group is None

It is possible to create a regexp with named groups which evaluates some groups values to `None` like in following example:

    import re

    rx_pattern = r'([^(]|^)(http|https)://(?:www.|)vimeo\.com/(?P<vimeoid>\d+)(?:\[(?P<width>\d+|),\s*,\s*(?P<height>\d+|)\]|)\S*'
    test_str = "http://vimeo.com/45678967234"

    rx = re.compile(rx_pattern)
    matches = rx.search(test_str)
    print matches.groups()   # >>> ('', 'http', '45678967234', None, None)

In this case serializator failed when trying to find something inside string which is `None`