incr/decr behavior is different from documentation

[elbaro]

    async def _incr_decr(
        self, conn: Connection, command: bytes, key: bytes, delta: int
    ) -> Optional[int]:
        cmd = b"%b %b %a\r\n" % (command, key, delta)
        resp = await self._execute_simple_command(conn, cmd)
        if not resp.isdigit() or resp == const.NOT_FOUND:
            raise ClientException(
                'Memcached {} command failed'.format(str(command)), resp)
        return int(resp) if resp.isdigit() else None

This line return int(resp) if resp.isdigit() else None never returns None because an exception is raised first if it's not digits.

Also from the docstring of incr/decr, resp == const.NOT_FOUND should return None but here it raises ClientException.

"""
        :return: ``int``, new value of the item's data,
        after the increment or ``None`` to indicate the item with
        this value was not found
"""