General: Ensure `get_tag_regex()` always returns a string, to match the documented value.

Props subrataemfluence.
Fixes #45643.

git-svn-id: https://develop.svn.wordpress.org/trunk@47430 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -6548,7 +6548,7 @@ function wp_auth_check( $response ) {
  */
 function get_tag_regex( $tag ) {
 	if ( empty( $tag ) ) {
-		return;
+		return '';
 	}
 	return sprintf( '<%1$s[^<]*(?:>[\s\S]*<\/%1$s>|\s*\/>)', tag_escape( $tag ) );
 }