think about the "null" object representation

[rgerhards]

currently (inherited from json-c), the null object is implemented differently from any other objects. It's not a struct json_object (ptr), but rather C NULL. This is inconsistent, and causes some trouble with the API (e.g. json_object_object_get(), which was deprecated for that reason). This also means that all callers need to handle the null object differently.

We should at least consider the represent the null object differently. A natural choice would be to do it just like with any other object with only the type set to fjson_type_null.

see also json-c/json-c#226

[ilyash-b]

I strongly recommend that JSON "null" will not be represented as C NULL. They are semantically different things. Keeping it C NULL will perpetuate all the regular problem that come from semantic misrepresentation: more code and more convoluted code to handle that object.