PHP 8 warning in menu include

Problem/Motivation

In includes/menu.inc, Radix adds a unique class based on the menu link title:

  // Add a unique class using the title.
  $title = strip_tags($element['#title']);
  $element['#attributes']['class'][] = 'menu-link-' . drupal_html_class($title);

There are cases where $element['#title'] is null. This is the result of a wonky condition that I can't reliably reproduce, but when it happens, there is a PHP 8.1 warning.

Steps to reproduce

See above.

Proposed resolution

Cast the argument to string. Or alternatively, check if the argument is null, and if it is, skip adding the class. Casting to string would mean that the end result was the same as on previous versions of PHP, which is probably the best for backward compatibility.

Remaining tasks

Patch and test.

User interface changes

None.

API changes

None.

Data model changes

None.