`lljson.encode()` should not encode NaN as illegal `NaN` literal

[HaroldCindy]

Many JSON parsers support NaN literals, but JSON.parse() in JS environments does not. JSON.stringify([NaN]) returns [null], so just do that. lljson.slencode() is available for cases where we care about round-tripping.

[Martin-Pitt]

The ECMAScript specification is the official standard where JSON comes from to begin with. Since there are some similarities between Lua and JavaScript (e.g. prototypal, number primitive, etc), it probably matters more here given the context for special numbers: JSON.stringify

Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are represented as the String value "null".