Adds empty sidebar to empty pages.

What does this MR do?

Fixes but with wiki sidebar not visible when no articles.

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Fixes: #38919 (closed)

cc @sarrahvesselov @tauriedavis I am not sure that an empty sidebar is the answer but I don't think an invisible one is the answer either. I think the sidebar should say something like "no articles yet"... If we make it invisible that might be like the dashboard of your car being hidden when it has nothing to show. Would be confusing.