Using a custom serializer seems to break downstream projects that also register serializers for EntityModel. We're now using extra methods on EntityModel itself that do the trick as well.
We now keep internal methods in EntityModel that are configured using Jackson annotations so that it'd populate the content with a Map in case Map<String, Content> is defined as payload type.
Significant rework of the nullability annotation introduction to not produce any warnings anymore. Added nullability annotations where still missing. Removed the ones that were added erroneously.
Tightened nullability contracts in a couple of places, most prominently LinkRelationProvider, that now uses a LookupContext as plugin delimiter so that implementations can selectively be looked up for item or collection link relation lookup.
Removed obsolete declaration of Findbugs dependency.
