update stubs for path and creation_date in torrent.pyi

Fixing a couple details I missed: - `path.setter` accepts None https://github.com/rndusr/torf/blob/c26f3b3210bba352ca924d7f7ccef7dc89c8c587/torf/_torrent.py#L177-L182 - `creation_date` property can return None https://github.com/rndusr/torf/blob/c26f3b3210bba352ca924d7f7ccef7dc89c8c587/torf/_torrent.py#L942-L957