UnexpectedValueException for links without routes

Problem/Motivation

In 🐛 Update Layout Builder panel size Active , Patternkit added a link-alter hook to customize links with route "layout_builder.add_block". If that hook tries to process a link without a route, Drupal logs an error like this:

UnexpectedValueException: https://www.drupal.org has no corresponding route. in Drupal\Core\Url->getRouteName() (line 574 of /var/www/web/core/lib/Drupal/Core/Url.php).

Steps to reproduce

- Install Drupal 11.2 with standard profile
- Install Patternkit (latest release 9.1.x-dev)
- In Drupal, add a menu item to the main menu (e.g., at /admin/structure/menu/manage/main/add).
- Use this external URL for that link: "https://www.drupal.org"
- Save it
- Go to /admin/reports/dblog.
- Notice that you see an error message of type "patternkit", with the error metioned above.

Proposed resolution

See proposed MR.

Remaining tasks

Update tests?