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.

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