fix static test failure (#22833: Short-term admin accounts)

Author: lfolco

app/code/Magento/Security/Api/Data/UserExpirationInterface.php

@@ -53,7 +53,7 @@ public function setUserId($userId);
     /**
      * Retrieve existing extension attributes object or create a new one.
      *
-     * @return UserExpirationExtensionInterface|null
+     * @return UserExpirationExtensionInterface
      */
     public function getExtensionAttributes();