Cast to string when passing to MailFormatHelper::htmlToText()

[claudiu-cristea]

When running drush core:requirements on PHP 8.1 I'm getting:

[error]  Message: /Deprecated function/: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\Component\Utility\Html::escape() (line 424 of /core/lib/Drupal/Component/Utility/Html.php/).
Drupal\Component\Utility\Html::escape(NULL) (Line: 262)
Drupal\Component\Render\FormattableMarkup::placeholderEscape(NULL) (Line: 232)
Drupal\Component\Render\FormattableMarkup::placeholderFormat('%profile_name (%profile-%version)', Array) (Line: 195)
Drupal\Core\StringTranslation\TranslatableMarkup->render() (Line: 15)
Drupal\Core\StringTranslation\TranslatableMarkup->__toString()
str_replace('', '', Object) (Line: 69)
Drupal\Component\Utility\Xss::filter(Object, Array) (Line: 117)
Drupal\Core\Mail\MailFormatHelper::htmlToText(Object) (Line: 23)
Drush\Drupal\DrupalUtil::drushRender(Object) (Line: 125)
Drush\Drupal\Commands\core\DrupalCommands->requirements(Array)
...

The issue is that we're passing TranslatableMarkup instances.