Formatting: Prevent `wp_targeted_link_rel()` adding an empty attribute.

Bypass adding a `rel` attribute when the `wp_targeted_link_rel` filter returns an empty string or other falsy result.

Props mcmwebsol, spartank, meatman89fs.
Fixes #45352.

Built from https://develop.svn.wordpress.org/trunk@44691


git-svn-id: http://core.svn.wordpress.org/trunk@44522 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/formatting.php

@@ -3065,6 +3065,11 @@ function wp_targeted_link_rel_callback( $matches ) {
 	 */
 	$rel = apply_filters( 'wp_targeted_link_rel', 'noopener noreferrer', $link_html );
 
+	// Avoid additional regex if the filter removes rel values.
+	if ( ! $rel ) {
+		return "<a $link_html>";
+	}
+
 	// Value with delimiters, spaces around are optional.
 	$attr_regex = '|rel\s*=\s*?(\\\\{0,1}["\'])(.*?)\\1|i';
 	preg_match( $attr_regex, $link_html, $rel_match );

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-beta2-44690';
+$wp_version = '5.1-beta2-44691';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.