Access to systemAdminMenuBlockPage is denied if it contains links using the url attribute

Problem/Motivation

The current access check implemented on SystemAdminMenuBlockAccessCheck doesn't work if the configured menu item uses the attribute url.

Steps to reproduce

1. Install the latest Drupal version.
2. Create a module test_link and implement the following route on test_link.routing.yml:
   

   test_link.admin:
     path: "/admin/test-link"
     defaults:
       _controller: '\Drupal\system\Controller\SystemController::systemAdminMenuBlockPage'
       _title: "Test Link"
     requirements:
       _permission: "access content"
   

3. Implement the following menu links on `test_link.links.menu.yml`:
   

   test_link.admin:
     title: "Test Link Admin"
     route_name: "test_link.admin"
     parent: system.admin_content
   
   test_link.admin.content:
     title: "Content List"
     url: "internal:/admin/content"
     description: "Content listing"
     parent: test_link.admin
   

4. Enable the module
5. With an admin user, access the page /admin/test-link

Proposed resolution

We need to check if the menu link doesn't have a route associated with it and implement a logic to validate the cases where the link has the url attribute.

Remaining tasks

Implement the new logic;
Implement the automated tests;

User interface changes

N/A

Introduced terminology

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A