Use Case

As an EJSON user,
I want `undefined` to be converted to `{ $undefined: true }`
So that we are EJSON spec compliant.

User Experience

• For as long as I can tell, EJSON has converted `undefined` to `null`.  This aligns with our BSON serializer's behavior.

Dependencies

• n/a

Risks/Unknowns

• Is it worth making this change at all?  This behavior is unlikely to be changed in our serializer.

Acceptance Criteria

Implementation Requirements

• Serialize/stringify undefined as `{ $undefined: true }`.

Testing Requirements

• There don't appear to be spec tests for this behavior - should we add them?

Documentation Requirements

• DOCSP ticket, API docs, etc

Follow Up Requirements

• additional tickets to file, required releases, etc
• if node behavior differs/will differ from other drivers, confirm with dbx devs what standard to aim for and what plan, if any, exists to reconcile the diverging behavior moving forward