Explicitly setting a property of a record to "undefined" and saving it to the database stores null instead of undefined

New Issue Checklist

Issue Description

Recently upgraded from Parse 4.10 to Parse 6.5 and noticed the following change in behavior which i assume is a bug.

When storing a record in the database that has a property explicitly set to "undefined", a null value is stored.
I.e. the behavior is different than if you do not set a property on the object as save it.
Again, in Parse 4.10, storing a property explicitly set to undefined, did not touch the property (i.e. left it to whatever it was before, for example undefined).

Steps to reproduce

Use the JS SDK.
Create an empty record in the parse dashboard.
explicitly set a property to "undefined" of a record using the JS sdk.
store the record.
look at the database in the dashboard and see that the value is Null instead of undefined

Actual Outcome

Null stored in the property of the db

Expected Outcome

The property not to be updated

Environment

Server

• Parse Server version: 6.5
• Operating system: Mac OSX Sonoma 14.4/Node
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): Local

Database

• System (MongoDB or Postgres): MongoDB
• Database version: 6.0.15
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): Atlas

Client

• SDK (iOS, Android, JavaScript, PHP, Unity, etc): JavaScript
• SDK version: 4.1

Logs

Reproduce the behavior with Parse Server 6.5, the JavaScript SDK, and MongoDB by explicitly setting a record property to undefined and saving it. Compare this with leaving the property unset, then trace the server's save handling to identify why undefined becomes null; done means the property remains unchanged and regression coverage verifies both cases.