When a date is set to null in mongoDB, parse server returns a Date set at 1970-01-01

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

Parse server replaces null value with a Date set at 1970-01-01 when sending back the data for a query.

Steps to reproduce

In any collection with a Date field, set the value to null (either directly in the MongoDB or through the parse sdk, e.g. object.set('date', null))

Actual Outcome

I get a Date instead of null, set to 1970-01-01. (just like new Date(null)).

Expected Outcome

I expect the value to be null.

Environment

Server

• Parse Server version: 4.10.3
• Operating system: Docker image node:14-alpine
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): AWS

Database

• System (MongoDB or Postgres): MongoDB
• Database version: 4.4.8
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): MongoDB Atlas

Client

• SDK (iOS, Android, JavaScript, PHP, Unity, etc): iOS, Android, JavaScript, cURL
• SDK version: not relevant. The network panel in Chrome shows the json sent by parse server already has the issue.

Logs

No source file or test is named in the issue. Reproduce the query response with a MongoDB-backed Parse Server using a Date field set to null, then trace the response serialization; done means the API returns null rather than a 1970-01-01 Date and a regression test covers that case.