Drupal\Core\Controller\TitleResolver::getTitle() can result in PHP 8.1 deprecation notices due to NULL being set as placeholder values

Problem/Motivation

\Drupal\Core\Controller\TitleResolver::getTitle() adds all the raw parameters of a route to the placeholder arguments array by doing:

      if (($raw_parameters = $request->attributes->get('_raw_variables'))) {
        foreach ($raw_parameters->all() as $key => $value) {
          $args['@' . $key] = $value;
          $args['%' . $key] = $value;
        }
      }

If a raw parameter has a NULL value this will be added to the $args array. In PHP 8.1 this will trigger deprecation notices because we'll end up passing these NULL values to a built-in function that expects a string - namely htmlspecialchars().

Steps to reproduce

Proposed resolution

This deprecation should be fixed where the values are assigned - ie. \Drupal\Core\Controller\TitleResolver::getTitle(). It is tempting to fix in \Drupal\Component\Utility\Html::escape(), \Drupal\Component\Render\FormattableMarkup::placeholderEscape() or \Drupal\Component\Render\FormattableMarkup::placeholderFormat() - however these method lack the context to determine where a NULL value is an unexpected error and we should work why there is no string or it is safe to replace with an empty string.

Additionally fixing it this will allow us to add typehints to \Drupal\Component\Render\FormattableMarkup::placeholderEscape() and \Drupal\Component\Utility\Html::escape() to improve the robustness of our code.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet