Fix the object null property-read bug when stringify the null node.

[dota17]

When we run

a = YAML.parseDocument("null")
console.log(YAML.stringify(a))

Get an exception：

   tagObj = value instanceof Map ? map : value[Symbol.iterator] ? seq : map;
                                               ^

TypeError: object null is not iterable (cannot read property Symbol(Symbol.iterator))
    at Object.createNode (c:\Users\l00498592\Desktop\workdata\yaml-master\dist\resolveSeq-6b2dc4ac.js:81:48)
    at Document.createNode (c:\Users\l00498592\Desktop\workdata\yaml-master\dist\Document-88366d37.js:542:29)
    at new Document (c:\Users\l00498592\Desktop\workdata\yaml-master\dist\Document-88366d37.js:509:28)
    at Object.stringify (c:\Users\l00498592\Desktop\workdata\yaml-master\dist\index.js:62:10)
    at Object.<anonymous> (c:\Users\l00498592\Desktop\workdata\yaml-master\test.js:74:18)
    at Module._compile (internal/modules/cjs/loader.js:1137:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1157:10)
    at Module.load (internal/modules/cjs/loader.js:985:32)
    at Function.Module._load (internal/modules/cjs/loader.js:878:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)

[eemeli]

Thank you! That's clearly not a usage pattern that I'd considered, and your fix looks just right for it. :)