Enable nullable for MediaBrowser.Providers project

[martinek-stepan]

Changes

Enable nullable on the project level and reflect changes in code.

Issues

Part of issue #2768

[daullmer]

We are currently removing the metadata providers from jellyfin/jellyfin into seperate repositories (see #4530 #4531 #4532). But we still need a method of installing them during the initial setup. That's why the PRs are not merged. The new plugins already have nullable enabled (https://github.com/jellyfin/jellyfin-plugin-omdb, https://github.com/jellyfin/jellyfin-plugin-tmdb, https://github.com/jellyfin/jellyfin-plugin-musicbrainz). I think we should wait merging this PR till the providers are removed, if that's okay for you

[stale]

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[1337joe]

Original issue (#2768) fixed by #6737, closing this as OBE