Resolve "Error 500 viewing notes with anonymous user"

What does this MR do?

When a group has a pending email invite for an owner or master, with no associated user ID, we'd consider the anonymous user an owner.

1. Don't do that.
2. Add a bit of extra safety to the methods on Group.

Are there points in the code the reviewer needs to double check?

I couldn't find an existing scope for this, so I just added where.not(user_id: nil).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #35444 (closed).