fix: change pid return type from undefined to be null

src/client/stdio.test.ts

@@ -64,7 +64,7 @@ test("should return child process pid", async () => {
   const client = new StdioClientTransport(serverParameters);
 
   await client.start();
-  expect(client.pid).toBeDefined();
+  expect(client.pid).not.toBeNull();
   await client.close();
-  expect(client.pid).toBeUndefined();
+  expect(client.pid).toBeNull();
 });