fix: fix empty content sidebar panel

[shreya0204]

What?

This fix hides the Content Panel when editing a template if no content is available.

Why?

Fixes: #64472

How?

Added a check if content is available, if not then returning null

Testing Instructions

1. Activate Twenty Twenty-Four.
2. Go to Appearance > Editor.
3. Open the editor, editing for example Blog Home.
4. Delete the header and footer template parts from the template.
5. Open the settings sidebar.
6. The Template Panel should be hidden

Testing Instructions for Keyboard

Screenshots or screencast

https://www.loom.com/share/a4b9f6cb7a644b8489d9669b5354ad5a?sid=2508bc29-63ea-4935-8ad5-16aa6c98665d

[github-actions]

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Co-authored-by: shreya0204 <[email protected]>
Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: hanneslsm <[email protected]>
Co-authored-by: t-hamano <[email protected]>
Co-authored-by: jasmussen <[email protected]>
Co-authored-by: richtabor <[email protected]>
Co-authored-by: carolinan <[email protected]>

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[t-hamano]

LGTM! It's working as expected 👍