minor #9625 [HttpFoundation] Deprecate Request::getSession() when Request::hasSession() is false (snoek09)

This PR was squashed before being merged into the master branch (closes #9625).

Discussion
----------

[HttpFoundation] Deprecate Request::getSession() when Request::hasSession() is false

This fixes #9457.

Commits
-------

b8f7627 [HttpFoundation] Deprecate Request::getSession() when Request::hasSession() is false

Author: javiereguiluz

components/http_foundation.rst

@@ -235,6 +235,12 @@ the
 method tells you if the request contains a session which was started in one of
 the previous requests.
 
+.. versionadded:: 4.1
+    Using :method:`Symfony\\Component\\HttpFoundation\\Request::getSession()`
+    when no session has been set was deprecated in Symfony 4.1. It will throw
+    an exception in Symfony 5.0 when the session is ``null``. Check for an existing session
+    first by calling :method:`Symfony\\Component\\HttpFoundation\\Request::hasSession()`.
+
 Accessing ``Accept-*`` Headers Data
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

security/impersonating_user.rst

@@ -209,11 +209,15 @@ you switch users, add an event subscriber on this event::
     {
         public function onSwitchUser(SwitchUserEvent $event)
         {
-            $event->getRequest()->getSession()->set(
-                '_locale',
-                // assuming your User has some getLocale() method
-                $event->getTargetUser()->getLocale()
-            );
+            $request = $event->getRequest();
+
+            if ($request->hasSession() && ($session = $request->getSession)) {
+                $session->set(
+                    '_locale',
+                    // assuming your User has some getLocale() method
+                    $event->getTargetUser()->getLocale()
+                );
+            }
         }
 
         public static function getSubscribedEvents()