Enable Absent "universe" and "address" in updateFixtures JSON

[Firionus]

Currently, when a fixture is updated and there is no change to the "universe" or "address" field, a null value is serialized. This is valid, but it would be nicer if the fields were completely absent.

This is currently not possible in Klaxon so we first need to add a feature upstream. While there already is the "serializeNull" feature, it does not work if we need to use a custom converter (as we have to in this case).

This issue is very low priority as the overhead is not too large, patch changes don't occur often and we may change JSON package in the future anyway.