Embeds: Change attachment metadata condition to prevent a warning in the embeds template.

See #35237.
Fixes #36838 for trunk.

git-svn-id: https://develop.svn.wordpress.org/trunk@37452 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme-compat/embed-content.php

@@ -27,7 +27,7 @@
 		$image_size   = 'full'; // Fallback.
 
 		$meta = wp_get_attachment_metadata( $thumbnail_id );
-		if ( is_array( $meta ) ) {
+		if ( ! empty( $meta['sizes'] ) ) {
 			foreach ( $meta['sizes'] as $size => $data ) {
 				if ( $data['width'] / $data['height'] > $aspect_ratio ) {
 					$aspect_ratio = $data['width'] / $data['height'];