Always pass a string as third argument (#189) (#190)

The type hint is string.

Author: ElectricMaxxx

src/Functional/BaseTestCase.php

@@ -195,6 +195,6 @@ protected function assertResponseSuccess(Response $response)
             $exception = $result->item(0)->nodeValue;
         }
 
-        $this->assertEquals(200, $response->getStatusCode(), $exception ? 'Exception: "'.$exception.'"' : null);
+        $this->assertEquals(200, $response->getStatusCode(), $exception ? 'Exception: "'.$exception.'"' : '');
     }
 }