feat: support nullable embedded entities

[test137E29B]

Support nullable embedded fields such that embedded documents in MongoDB can correctly be returned as null for a subdocument that is explicitly nullable.

Closes: #3913

Description of change

This change adds to the ColumnEmbeddedOptions the nullable field, thus allowing subdocuments in MongoDB to be null explicitly. This ensures they're returned correctly from the database in the case a null is set. This is useful when a developer has a required key that can be set to null - currently the key is not returned at all.

I saw this in #3913 though this was reverted as it previously changed all nested fields to null, which caused some problems I imagine for people. This method of implementing this change should not cause problems or any breaking changes and is entirely optional.

I have locally tested this with MongoDB, but not a SQL provider.

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run format to apply prettier formatting
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md

[test137E29B]

Would love if someone could add support for this in SQL providers, since my changes don't seem to have worked. I can't imagine it's not too hard.

Alternatively, I can just add a note that this will only work for MongoDB.

Open to opinions on this 😃