Preserve undefined vs null in encoders

[bhelx]

In the json encoder / decoder, we were essentially re-writing undefineds with null. This should preserve the value as null or undefined in the falsey case.

> x = undefined
> 
> x == null ? x : x.toString()
> 
> x = null
> null
> x == null ? x : x.toString()
> null
> x = 123
> 123
> x == null ? x : x.toString()
> '123'

[chrisdickinson]

LGTM!