NULL should be lowercased

[Swimburger]

When a value is null, the JsonViewer renders it as NULL instead of null.
It's pedantic, but NULL isn't valid and inconsistent with the actual value, so I think it should be all lowercased null.

Is there a way to override this behavior? I'd be happy to make a PR for this if you agree.

[YYsuni]

Hi, you could give this a try: https://github.com/YYsuni/react18-json-view