`null` semantics in JSON serialization

In the JSON serialization, is there an intended difference in semantics between:
* the absence of a property
* the property set to `null`?
* in the case of objects/arrays, empty objects or arrays?

As a corollary, can all optional properties be set to `null`, or only some of them?

No file, test, or entry point is named. First compare current JSON serialization behavior with existing OpenTimelineIO format documentation and examples, then resolve and document the semantics of absent, null, and empty object or array values, including which optional properties may be null.

Written by the indexing model from the issue text.