Make null behaviour configurable

[effad]

As detailed in Issue #296, the current handling of null values in Java objects that are converted to JSON is controversial.

An attempt to provide a way to configure the behaviour was given in PR #338 but ultimately rejected due to API concerns (boolean parameter to trigger behaviour).

Meanwhile, JsonParserConfiguration has been introduced, which would allow to make the null behaviour configurable in an elegant way by extending JsonParserConfiguration.

If this seems feasible, I would be willing to provide a PR for this.

[stleary]

@effad Please see #296 for the latest comments.
Closing this issue in favor of the original discussion in #296