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

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