Re-check JSON null normalization in lemmyv1 adapter

[aeharding]

The lemmyv1 adapter currently normalizes JSON null to undefined in a few places because the Lemmy 1.0 beta wire format returns null for absent optional fields rather than omitting them.

• nullToUndef helper used on notification.{comment_id,modlog_id,post_id,private_message_id} (src/providers/lemmyv1/compat.ts:196-199)
• == null (instead of === undefined) checks against PostActions.{vote_is_upvote,read_comments_at} (src/providers/lemmyv1/compat.ts:270,289)

These workarounds papered over schemas that expect undefined. Once Lemmy stabilizes 1.0, recheck whether the server actually emits null for these fields and either:

1. Update our schemas to accept null explicitly (z.nullish) and drop the helpers, or
2. Confirm the server omits the fields and remove the normalization.

[Nutomic]

These two structs were missing the annotation for #[skip_serializing_none]. If you notice any more problems like this, please open an issue in the Lemmy repo directly.

Fix here: LemmyNet/lemmy#6528