[TwigBridge] Make AppVariable check if security.context exists

If security isn't configured in the application, the `security.context` service, nor the `security.token_storage` service exists.
Therefore, if a third-party bundle relies on the app.user check in Twig templates, an exception was thrown about asking for an non-existing service.
Instead, this change check if the `security.context` actually exists before trying to use it, and return null otherwise.

src/Symfony/Bridge/Twig/AppVariable.php

@@ -74,7 +74,9 @@ class AppVariable
             throw new \RuntimeException('The "app.security" variable is not available.');
         }
 
-        return $this->container->get('security.context');
+        if ($this->container->has('security.context')) {
+            return $this->container->get('security.context');
+        }
     }
 
     /**
@@ -89,6 +91,8 @@ class AppVariable
         if (null === $this->tokenStorage) {
             if (null === $this->container) {
                 throw new \RuntimeException('The "app.user" variable is not available.');
+            } elseif (!$this->container->has('security.context')) {
+                return;
             }
 
             $this->tokenStorage = $this->container->get('security.context');