fix error when season folder not exists

[fiochen]

#126 (comment)

10.9.7版本中SeriesFolder/episode_file会异常：

[2024-07-14 15:51:20.314 +08:00] [INF] [98] MediaBrowser.Providers.TV.SeriesMetadataService: Creating Season "第 1 季" entry for "败犬女主太多了！"
[2024-07-14 15:51:20.318 +08:00] [ERR] [98] MediaBrowser.Providers.TV.SeasonMetadataService: Error in "Bangumi"
System.ArgumentNullException: Value cannot be null. (Parameter 'path')
   at System.ArgumentNullException.Throw(String paramName)
   at System.ArgumentNullException.ThrowIfNull(Object argument, String paramName)
   at System.ArgumentException.ThrowNullOrEmptyException(String argument, String paramName)
   at Emby.Server.Implementations.Library.LibraryManager.FindByPath(String path, Nullable`1 isFolder)
   at Jellyfin.Plugin.Bangumi.Providers.SeasonProvider.GetMetadata(SeasonInfo info, CancellationToken token) in /app/Jellyfin.Plugin.Bangumi/Providers/SeasonProvider.cs:line 49
   at MediaBrowser.Providers.Manager.MetadataService`2.ExecuteRemoteProviders(MetadataResult`1 temp, String logName, Boolean replaceData, TIdType id, IEnumerable`1 providers, CancellationToken cancellationToken)