Code Modernization: Check the return type of parse_url() in ms_cookie_constants().

As per the PHP manual:
> If the `component` parameter is omitted, an associative array is returned.
> If the `component` parameter is specified, `parse_url()` returns a string (or an int, in the case of `PHP_URL_PORT`) instead of an array. If the requested component doesn't exist within the given URL, `null` will be returned.

Reference: [https://www.php.net/manual/en/function.parse-url.php#refsect1-function.parse-url-returnvalues PHP Manual: parse_url(): Return Values]

It is entirely possible for the `siteurl` option to not have a "path" component.

In PHP 8.1, this would lead to a `trim(): Passing null to parameter #1 ($string) of type string is deprecated` notice.

Changing the logic around and adding validation for the return type value of `parse_url()` prevents that.

As this function is declaring global constants, adding tests for this change is not really an option without potentially affecting other tests.

Follow-up to [51606], [51622], [51626], [51629].

Props jrf, hellofromTonya, SergeyBiryukov.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51630 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/ms-default-constants.php

@@ -68,7 +68,8 @@ function ms_cookie_constants() {
 	 * @since 2.6.0
 	 */
 	if ( ! defined( 'ADMIN_COOKIE_PATH' ) ) {
-		if ( ! is_subdomain_install() || trim( parse_url( get_option( 'siteurl' ), PHP_URL_PATH ), '/' ) ) {
+		$site_path = parse_url( get_option( 'siteurl' ), PHP_URL_PATH );
+		if ( ! is_subdomain_install() || is_string( $site_path ) && trim( $site_path, '/' ) ) {
 			define( 'ADMIN_COOKIE_PATH', SITECOOKIEPATH );
 		} else {
 			define( 'ADMIN_COOKIE_PATH', SITECOOKIEPATH . 'wp-admin' );