Ensure proper `null` (as empty) handling for `AtomicReference` wrt serialization inclusion

As per proposal on dev mailing list, I think it makes sense to make all referential types, like:
- `AtomicReference`
- JDK Optional
- Guava Optional

work so that "absent" case (where non-null reference value refers to `null` or "absent") will match "empty" and "default" inclusion criteria, but NOT "null", for purposes of serialization filtering.
