fix!: StickerPack.cover_sticker_id and .banner_asset_id are optional

[shiftinv]

Summary

The cover_sticker_id and banner_asset_id fields in the sticker pack data are marked as optional in the API docs
, but have so far been treated as required.

This was fine in all but the latest "Snowsgiving Surprise Pack", which does not have a banner, breaking Client.fetch_premium_sticker_packs.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running task lint
  • I have type-checked the code by running task pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)