Nested pointer updates don't work

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version of Parse Server.

Issue Description

Updates to nested pointers don't work.

Steps to reproduce

1. Create a Parse object with a nested pointer:

const obj = new Parse.Object('GameScore'); // This object will include nested pointers
const ptr1 = new Parse.Object('GameScore');
await ptr1.save(); // Obtain a unique id
const ptr2 = new Parse.Object('GameScore');
await ptr2.save(); // Obtain a unique id
obj.set('data', { ptr: ptr1 });
await obj.save();

2. Update the nested pointer

obj.set('data.ptr', ptr2);
await obj.save();

3. Re-read the object's nested pointer field

const obj2 = await new Parse.Query('GameScore').get(obj.id);
expect(obj2.get('data').ptr.id).toBe(ptr2.id);

Actual Outcome

The update on the nested pointer does not take effect, so the assert fails.

Expected Outcome

The pointer should be updated as instructed.

Failing Test Case / Pull Request

• 🤩 I submitted a PR with a fix and a test case.
• 🧐 I submitted a PR with a failing test case.

Environment

Server

• Parse Server version: master as of 20 May 2021
• Operating system: Ubuntu 20.04
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): local

Database

• System (MongoDB or Postgres): MongoDB
• Database version: v4.4.6
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): local

Client

• SDK (iOS, Android, JavaScript, PHP, Unity, etc): JavaScript
• SDK version: 3.2.0

Logs

No implementation file or test path is named. Start with the JavaScript reproduction in the issue and trace how Parse Server persists updates to nested pointer fields in MongoDB. Done means re-reading the object shows data.ptr referring to ptr2, with a regression test covering the update.