minor #10717 unified return null usages (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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unified return null usages

| Q             | A
| ------------- | ---
| License       | MIT

This PR unifies the way we return `null` from a function or method:

 * always use `return;` instead of `return null;` (the current code base uses both);
 * never use `return;` at the end of a function/method.

Commits
-------

d1d569b unified return null usages

Author: fabpot

Firewall/ContextListener.php

@@ -171,7 +171,7 @@ private function refreshUser(TokenInterface $token)
                     $this->logger->warning(sprintf('Username "%s" could not be found.', $notFound->getUsername()));
                 }
 
-                return null;
+                return;
             }
         }
 

RememberMe/AbstractRememberMeServices.php

@@ -142,8 +142,6 @@ final public function autoLogin(Request $request)
         }
 
         $this->cancelCookie($request);
-
-        return null;
     }
 
     /**