Return footerReference element of *type_* or None if not present.

def get_footerReference(self, type_):
        """Return footerReference element of *type_* or None if not present."""
        path = "./w:footerReference[@w:type='%s']" % WD_HEADER_FOOTER.to_xml(type_)
        footerReferences = self.xpath(path)
        if not footerReferences:
            return None
        return footerReferences[0]