test: add null and empty token cases to logout test

Red phase: Logout should handle null and empty string tokens gracefully without throwing. Currently null causes TypeError.

backend/tests/Unit/Auth/UseCases/LogoutTest.php

@@ -50,4 +50,18 @@ class LogoutTest extends TestCase
 
         $this->assertTrue(true);
     }
+
+    public function testDoesNotThrowForNullToken(): void
+    {
+        $this->useCase->execute(null);
+
+        $this->assertTrue(true);
+    }
+
+    public function testDoesNotThrowForEmptyStringToken(): void
+    {
+        $this->useCase->execute('');
+
+        $this->assertTrue(true);
+    }
 }