Using revert in beforeSave triggers doesn't return the correct response #7838

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

As discussed here, using revert doesn't work due to revert removing ops for the key, so the keys' update won't be returned by the API.

I.e, for an update of a key foo, the server will respond:

{ 'foo' : 'bar' }

And the client will merge that with the existing object, so

{'name': 'f'} would become {'name': 'f', 'foo': 'bar'}

However, when revert is used, the server responds with {}, which gets merged with the clients' JSON of { 'foo' : 'bar' }, meaning the client thinks the key update was successful.

Steps to reproduce

it('should revert in beforeSave', async () => {
    Parse.Cloud.beforeSave('MyObject', ({object}) => {
      if (!object.existed()) {
        object.set('count', 0);
        return object;
      }
      object.revert('count');
      return object;
    });
    const obj = await new Parse.Object('MyObject').save();
    expect(obj.get('count')).toBe(0);
    obj.set('count', 10);
    await obj.save();
    expect(obj.get('count')).toBe(0); // test fails, JS SDK thinks count was successfully updated
    await obj.fetch();
    expect(obj.get('count')).toBe(0); // test passes
});

Actual Outcome

Keys are updated

Expected Outcome

Keys to be reverted

Environment

Server

• Parse Server version: alpha
• Operating system: macos
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): localhost

Database

• System (MongoDB or Postgres): mongo
• Database version: 5
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): local

Client

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

Logs