Make ShiyuDefense begin_time and end_time Optional

[Polygonalr]

hadal_begin_time and hadal_end_time are returned as null if the player has not challenge this season's Shiyu Defense yet.

{
  "retcode": 0,
  "message": "OK",
  "data": {
    "schedule_id": 0,
    "begin_time": "0",
    "end_time": "0",
    "rating_list": [],
    "has_data": false,
    "all_floor_detail": [],
    "fast_layer_time": 0,
    "max_layer": 0,
    "hadal_begin_time": null,
    "hadal_end_time": null
  }
}

This can cause get_shiyu_defense to fail Pydantic validation. This PR addresses this by making both ShiyuDefense.begin_time and ShiyuDefense.end_time Optionals.

[seriaati]

Thanks for the PR, can you fix the lint issues?