Warn about empty templates in the frontend

[vcanales]

Proposes adding a warning for logged-in users when navigating to a page or post rendered through an empty template. The warning explains why the website's visitors might encounter a blank page and points the logged-in user to the Site Editor to fix the template by resetting it or adding content to it.

Related issue: https://github.com/WordPress/gutenberg/issues/64027

Trac ticket:

Screenshot

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Props vcanales, mikachan, peterwilsoncc, richtabor, get_dave, mrfoxtalbot, matveb, arielmaidana, seifradwane, annezazu.

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[mrfoxtalbot]

Thank you for working on this @vcanales, I added a comment with two minor suggestions https://github.com/WordPress/gutenberg/issues/64027#issuecomment-2385791997

[mrfoxtalbot]

Can I ask you, @cbravobernal, or maybe @getdave to review this? I'd love for this change to make it to the 6.7 in time. Thanks a bunch.

[mikachan]

Thanks for all the feedback, @richtabor 🙇 I think I've addressed all your comments, this is ready for another review.

[getdave]

@mikachan Let me know when you are ready to commit this and we can pair on commit to WP Core.

[getdave]

@mikachan This now needs a commit. A good opportunity for you. LMK if you want to run through this.

[mikachan]

Committed to Core in core.trac.wordpress.org/changeset/59449.