May adapter input / output be null?

Please add information to the JSON-B Spec / JavaDocs explicitly declaring how to handle `null` in `JsonbAdapter` implementations:
* MUST a `JsonbAdapter` implementation expect to receive `null`?
* MAY a `JsonbAdapter` implementation return `null`?

Locate the JSON-B specification and JavaDocs for JsonbAdapter, then read the existing adapter contract and its treatment of null values. Document whether implementations must accept null and may return null, with wording that makes both requirements unambiguous; the work is done when the spec and JavaDocs agree.

Written by the indexing model from the issue text.