Enqueued Assets Site Health test not fully accounting for async CSS

See [support topic](https://wordpress.org/support/topic/false-positive-error-in-enqueued-assets-health-check/).

Given a site with the following HTML:

```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:

<img width="634" height="532" alt="Image" src="https://github.com/user-attachments/assets/634d1e3c-72cd-4326-b046-ee3b754deb80" />

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

https://github.com/WordPress/performance/blob/f7846e4d3d11c0ae3bac04926c71904e8d299c76/plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php#L110-L132

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.


	} 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,
	);
	}

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Enqueued Assets Site Health test not fully accounting for async CSS #2278

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Enqueued Assets Site Health test not fully accounting for async CSS #2278

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