Embeds: Preserve site icon fallback URL

[dhrupo]

Trac ticket: https://core.trac.wordpress.org/ticket/65098

Summary

When a site icon option is set but wp_get_attachment_image_url() cannot resolve a URL, the provided fallback URL should be preserved instead of being replaced with false.

Testing Instructions

1. Set the site_icon option to an invalid attachment ID.
2. Call get_site_icon_url( 32, "https://example.org/fallback.png" ).
3. Confirm the fallback URL is returned.
4. Run npm run test:php -- --filter test_get_site_icon_url.

[github-actions]

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Props dhrupo.

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[github-actions]

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[apermo]

i was puzzled at first place why the test covers the changes, may add some comments why the test actually tests correctly.

Suggested change

	update_option( 'site_icon', 999999 );
	// Set site_icon to random not existing attachment id.
	update_option( 'site_icon', 999999 );

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62502
GitHub commit: 68cbdc9

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.