bug #14816 [TwigBridge] Make AppVariable check if security.context exists (ogizanagi)

This PR was merged into the 2.7 branch.

Discussion
----------

[TwigBridge] Make AppVariable check if security.context exists

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

If security isn't configured in the application, neither the `security.context` service, nor the `security.token_storage` service exists.

Therefore, if a third-party bundle relies on the `app.user` or `app.security` (deprecated) check in Twig templates, an exception was thrown about asking for an non-existing service:

```yml
security: false
```

```twig
{% if app.user %}
    <div>
        ...
    </div>
{% endif %}
```
```
You have requested a non-existent service "security.context"
```

Instead, this patch checks if the `security.context` actually exists before trying to use it, and returns null otherwise.

Note that the **GlobalVariables** class, used for twig app globals in previous versions, and still used for PHP templating, behaves the same way.
So this is basically a BC break.

Commits
-------

ea71174 [TwigBridge] Make AppVariable check if security.context exists

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');