[9.x] Adjusts Request::getSession upgrade guide (#7600)

* Adjusts getSession upgrade notes

* Update upgrade.md

Co-authored-by: Taylor Otwell <taylor@laravel.com>

Author: nunomaduro

upgrade.md

@@ -543,7 +543,7 @@ public function flush($age = null);
 
 The `Symfony\Component\HttpFoundaton\Request` class that is extended by Laravel's own `Illuminate\Http\Request` class offers a `getSession` method to get the current session storage handler. This method is not documented by Laravel as most Laravel applications interact with the session through Laravel's own `session` method.
 
-The `getSession` method previously returned an instance of `Illuminate\Session\Store`; however, due to the Symfony 6.x release enforcing a return type of `Symfony\Component\HttpFoundation\Session\SessionInterface`, the `getSession` now correctly returns a `SessionInterface` implementation.
+The `getSession` method previously returned an instance of `Illuminate\Session\Store` or `null`; however, due to the Symfony 6.x release enforcing a return type of `Symfony\Component\HttpFoundation\Session\SessionInterface`, the `getSession` now correctly returns a `SessionInterface` implementation or throws an `\Symfony\Component\HttpFoundation\Exception\SessionNotFoundException` exception when no session is available.
 
 ### Testing