minor symfony#24316 Added null as explicit return type (?TokenInterface) (iltar)

This PR was merged into the 2.7 branch.

Discussion
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Added null as explicit return type (?TokenInterface)

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#23882 (comment)
| License       | MIT
| Doc PR        | ~

This fixes the returntype in the `ContextListener` so it can be merged upwards.

/cc @chalasr

Commits
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1ba4dd9 Added null as explicit return type (?TokenInterface)

Author: nicolas-grekas

src/Symfony/Component/Security/Http/Firewall/ContextListener.php

@@ -174,7 +174,7 @@ protected function refreshUser(TokenInterface $token)
         }
 
         if ($userNotFoundByProvider) {
-            return;
+            return null;
         }
 
         throw new \RuntimeException(sprintf('There is no user provider for user "%s".', get_class($user)));