Return empty object when no fallback templates are found (wp/v2/templates/lookup)

[creativecoder]

Modifies the wp/v2/templates/fallback endpoint to return a 404 error when no templates are found.

Testing Instructions

• Activate a classic theme, such as twentyeleven
• Add a theme.json file to the theme with minimal content (e.g. { "version": 2 })
• Load a page in the editor
• You should see a request to /wp-json/wp/v2/templates/lookup respond with an empty object in the browser network tab
• There should not be any PHP warnings in the error log

Trac ticket: https://core.trac.wordpress.org/ticket/60909
Gutenberg PR: WordPress/gutenberg#60925

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[github-actions]

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[aaronjorbin]

Thanks! Merged in https://core.trac.wordpress.org/changeset/58079.