Returns response property and fills it if missing.

def fill_response_property(xml_root, name=None, value=None):
    """Returns response property and fills it if missing."""
    name, value = generate_response_property(name, value)
    response_property = None

    if xml_root.tag == "testsuites":
        response_property = _fill_testsuites_response_property(xml_root, name, value)
    elif xml_root.tag in ("testcases", "requirements"):
        response_property = _fill_non_testsuites_response_property(xml_root, name, value)
    else:
        raise Dump2PolarionException(_NOT_EXPECTED_FORMAT_MSG)

    return response_property