Code Modernization: Pass correct value to parse_url() in WP_Customize_Manager::get_return_url().

This particular code block only makes sense to run when `$this->return_url` is not null. Previously, it caused a "passing null to non-nullable" deprecation notice on PHP 8.1.

By moving the code into the `if ( $this->return_url )` condition block, the code will only be run when `$this->return_url` contains a non-falsey/non-null value.

No additional tests added as this issue was found via the existing tests for the function containing the bug.

This solves the following two PHP 8.1 test errors:
{{{
1) Tests_WP_Customize_Manager::test_return_url
parse_url(): Passing null to parameter #1 ($url) of type string is deprecated

/var/www/src/wp-includes/class-wp-customize-manager.php:4696
/var/www/tests/phpunit/tests/customize/manager.php:2975
/var/www/vendor/bin/phpunit:123

2) Tests_WP_Customize_Manager::test_customize_pane_settings
parse_url(): Passing null to parameter #1 ($url) of type string is deprecated

/var/www/src/wp-includes/class-wp-customize-manager.php:4696
/var/www/src/wp-includes/class-wp-customize-manager.php:4898
/var/www/tests/phpunit/tests/customize/manager.php:3085
/var/www/vendor/bin/phpunit:123
}}}

Follow-up to [46754].

Props jrf, costdev.
See #55656.

git-svn-id: https://develop.svn.wordpress.org/trunk@54135 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp-customize-manager.php

@@ -4686,6 +4686,21 @@ public function get_return_url() {
 
 		if ( $this->return_url ) {
 			$return_url = $this->return_url;
+
+			$return_url_basename = wp_basename( parse_url( $this->return_url, PHP_URL_PATH ) );
+			$return_url_query    = parse_url( $this->return_url, PHP_URL_QUERY );
+
+			if ( 'themes.php' === $return_url_basename && $return_url_query ) {
+				parse_str( $return_url_query, $query_vars );
+
+				/*
+				 * If the return URL is a page added by a theme to the Appearance menu via add_submenu_page(),
+				 * verify that it belongs to the active theme, otherwise fall back to the Themes screen.
+				 */
+				if ( isset( $query_vars['page'] ) && ! isset( $_registered_pages[ "appearance_page_{$query_vars['page']}" ] ) ) {
+					$return_url = admin_url( 'themes.php' );
+				}
+			}
 		} elseif ( $referer && ! in_array( wp_basename( parse_url( $referer, PHP_URL_PATH ) ), $excluded_referer_basenames, true ) ) {
 			$return_url = $referer;
 		} elseif ( $this->preview_url ) {
@@ -4694,21 +4709,6 @@ public function get_return_url() {
 			$return_url = home_url( '/' );
 		}
 
-		$return_url_basename = wp_basename( parse_url( $this->return_url, PHP_URL_PATH ) );
-		$return_url_query    = parse_url( $this->return_url, PHP_URL_QUERY );
-
-		if ( 'themes.php' === $return_url_basename && $return_url_query ) {
-			parse_str( $return_url_query, $query_vars );
-
-			/*
-			 * If the return URL is a page added by a theme to the Appearance menu via add_submenu_page(),
-			 * verify that it belongs to the active theme, otherwise fall back to the Themes screen.
-			 */
-			if ( isset( $query_vars['page'] ) && ! isset( $_registered_pages[ "appearance_page_{$query_vars['page']}" ] ) ) {
-				$return_url = admin_url( 'themes.php' );
-			}
-		}
-
 		return $return_url;
 	}