implement: move null/empty token guard into logout use case

Green phase: execute() accepts ?string, returns early for null or empty string, otherwise delegates to repository.

backend/app/Auth/UseCases/Logout/Logout.php

@@ -11,8 +11,12 @@ class Logout
     ) {
     }
 
-    public function execute(string $token): void
+    public function execute(?string $token): void
     {
+        if (! is_string($token) || $token === '') {
+            return;
+        }
+
         $this->sessionRepo->deleteByToken($token);
     }
 }