Add Site Health test to warn sites when fallback publisher logo will be used for Schema.org metadata

[westonruter]

Feature description

Users often reported structured data errors related to publisher.logo.url being empty. For this reason, we provide a fallback image amp-page-fallback-wordpress-publisher-logo.png. Nevertheless, this is not ideal.

There should be a Site Health test that warns administrators when amp_get_publisher_logo() returns an image other than the fallback of amp-page-fallback-wordpress-publisher-logo.png.

This field is empty for sites that do not have a Custom Logo or Site Icon set in the Customizer, or a custom amp_site_icon_url filter was not used. See logic here:

amp-wp/includes/amp-helper-functions.php

Lines 1593 to 1641 in afab6f0

	function amp_get_publisher_logo() {
	$logo_image_url = null;
	/*
	* This should be 60x600px rectangle. It *can* be larger than this, contrary to the current documentation.
	* Only minimum size and ratio matters. So height should be at least 60px and width a minimum of 200px.
	* An aspect ratio between 200/60 (10/3) and 600:60 (10/1) should be used. A square image still be used,
	* but it is not preferred; a landscape logo should be provided if possible.
	*/
	$logo_width = 600;
	$logo_height = 60;
	// Use the Custom Logo if set.
	$custom_logo_id = get_theme_mod( 'custom_logo' );
	if ( has_custom_logo() && $custom_logo_id ) {
	$custom_logo_img = wp_get_attachment_image_src( $custom_logo_id, [ $logo_width, $logo_height ], false );
	if ( ! empty( $custom_logo_img[0] ) ) {
	$logo_image_url = $custom_logo_img[0];
	}
	}
	// Try Site Icon if a custom logo is not set.
	$site_icon_id = get_option( 'site_icon' );
	if ( empty( $logo_image_url ) && $site_icon_id ) {
	$site_icon_src = wp_get_attachment_image_src( $site_icon_id, [ $logo_width, $logo_height ], false );
	if ( ! empty( $site_icon_src ) ) {
	$logo_image_url = $site_icon_src[0];
	}
	}
	/**
	* Filters the publisher logo URL in the schema.org data.
	*
	* Previously, this only filtered the Site Icon, as that was the only possible schema.org publisher logo.
	* But the Custom Logo is now the preferred publisher logo, if it exists and its dimensions aren't too big.
	*
	* @since 0.3
	*
	* @param string $schema_img_url URL of the publisher logo, either the Custom Logo or the Site Icon.
	*/
	$logo_image_url = apply_filters( 'amp_site_icon_url', $logo_image_url );
	// Fallback to serving the WordPress logo.
	if ( empty( $logo_image_url ) ) {
	$logo_image_url = amp_get_asset_url( 'images/amp-page-fallback-wordpress-publisher-logo.png' );
	}
	return $logo_image_url;
	}

The amp_get_publisher_logo() should perhaps take a $default arg that is set to amp-page-fallback-wordpress-publisher-logo.png when null. Then we can use this in the Site Health test by passing false to determine if a publisher logo is defined.

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