Re-check JSON null normalization in lemmyv1 adapter

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.