Change fallback for logged-out sidebar

What does this MR do and why?

Change fallback for logged-out sidebar

Currently the logged-out sidebar will fall-back to the "Your Work" panel. Unfortunately this doesn't make a lot of sense, because the "Your Work" panel relies on a user object existing. It also leads to exceptions currently.

Furthermore we add missing specs and insure that all the panels render fine, even if the user object is nil.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:super_sidebar_logged_out)

2. Make sure you are logged out.
3. Visit the in-product help page /help

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.