Use hasSession() over getSession() to check whether the session e…

…xists (mautic#11483)

As `getSession()` will throw an exception in Symfony 5 instead of `null` in Symfony 4

app/AppKernel.php

@@ -86,7 +86,7 @@ public function handle(Request $request, $type = HttpKernelInterface::MASTER_REQ
          * If we've already sent the response headers, and we have a session
          * set in the request, set that as the session in the container.
          */
-        if (headers_sent() && $request->getSession()) {
+        if (headers_sent() && $request->hasSession()) {
             $this->getContainer()->set('session', $request->getSession());
         }