Enqueued Assets Site Health test not fully accounting for async CSS

[westonruter]

See support topic.

Given a site with the following HTML:

<link rel='stylesheet' id='companion-bundle-css' href="" data-href='http://example.com/style.css' type='text/css' media='all' />

This is showing up as an error:

Note that the audit currently is accounting for async CSS whereby the media is set to something other than screen or all:

performance/plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php

Lines 110 to 132 in f7846e4

	} elseif ( 'LINK' === $tag ) {
	$rel = $processor->get_attribute( 'rel' );
	if ( 'stylesheet' !== strtolower( (string) $rel ) ) {
	continue;
	}
	$media = $processor->get_attribute( 'media' );
	if ( is_string( $media ) && 1 !== preg_match( '/^\s*(all|screen)\b/i', $media ) ) {
	continue;
	}
	$href = $processor->get_attribute( 'href' );
	if ( ! is_string( $href ) ) {
	continue;
	}
	$size = perflab_aea_get_asset_size( $href );
	$result['assets']['styles'][] = array(
	'src' => $href,
	'size' => is_wp_error( $size ) ? null : $size,
	'error' => is_wp_error( $size ) ? $size : null,
	);
	}

Normally this is accompanied by an onload attribute on the LINK tag to rewrite the media to screen from something like print when the stylesheet has loaded.

We should also account for this case where the href is empty.