fix type

Author: AbhiPrasad

packages/node/test/integrations/undici.test.ts

@@ -376,7 +376,7 @@ function setupTestServer() {
     res.connection.end();
   });
 
-  testServer.listen(18099, 'localhost');
+  testServer?.listen(18099, 'localhost');
 
   return new Promise(resolve => {
     testServer?.on('listening', resolve);