fix: allow null, string and string array as allowed tags (#50409)

* fix: allow null, string and string array as allowed tags

* Update Stringable.php

---------

Co-authored-by: Maarten Paauw <maarten.paauw@npo.nl>
Co-authored-by: Taylor Otwell <taylor@laravel.com>

Stringable.php

@@ -769,7 +769,7 @@ public function start($prefix)
     /**
      * Strip HTML and PHP tags from the given string.
      *
-     * @param  string  $allowedTags
+     * @param  string[]|string|null  $allowedTags
      * @return static
      */
     public function stripTags($allowedTags = null)