Uart.js/Puck.js should handle `undefined`

[gfwilliams]

http://forum.espruino.com/conversations/362281/#comment15956629

Puck/eval("undefined") breaks because what is returned isn't JSON.

Potentially we could actually use our own JSON(ish) parser that'll allow us to use E.toJS and parse {a:5} type things instead of requiring {"a":5}