Eh, dkjson implements null as an object with a metatable function that encodes it back as "null". Hopefully it's considered equal to itself in comparisons.

Dkjson is fast enough for most scripting purposes. OTOH cjson's userdata null is supported by some other libraries that deal with data structures.

Of course, there's a problem then that various libs may have their own nulls, not equal to each other. There's even a lib that tries to marry some of them.