Prevent error in OptOutManager when language is not a string (#20074)

Had the other day close to 80 fatal errors from a scanner where language was not a string which resulted in an error below

> preg_match(): Argument #2 ($subject) must be of type string, array given

plugins/CoreAdminHome/OptOutManager.php

@@ -558,7 +558,7 @@ public function getOptOutViewIFrame()
             }
         }
 
-        $language = Common::getRequestVar('language', '');
+        $language = Common::getRequestVar('language', '', 'string');
         $lang = APILanguagesManager::getInstance()->isLanguageAvailable($language)
             ? $language
             : LanguagesManager::getLanguageCodeForCurrentUser();