JsonbAdapter Javadocs should clarify handling of null

The Javadocs of `JsonbAdapter` should clarify whether...
* ...it is allowed to pass `null` as a parmeter (so the implementor knows whether this case is to be *handled*)
* ...it is allowed to return `null` as a return value (so the implementor knows whether implementing this is *legal*)

Start at the JsonbAdapter declaration and review its existing Javadocs. Clarify whether implementations may receive null parameters and whether they may return null, so the documented contract explicitly covers both cases.

Written by the indexing model from the issue text.