Load: empty input sring should return null, no undefined

[puzrin]

~~Ref: https://github.com/nodeca/js-yaml/issues/565#issuecomment-659696047~~

https://github.com/nodeca/js-yaml/issues/567#issuecomment-685070822

[puzrin]

Ups... closing after reference update

[ingydotnet]

Found a couple problems:

> require('js-yaml').load("")
undefined
> require('js-yaml').load("\n")
undefined
> require('js-yaml').load("\n\n")
null
> require('js-yaml').load("# hmm")
null

[rlidwka]

> require('js-yaml').loadAll("")
[]
> require('js-yaml').loadAll("\n")
[]
> require('js-yaml').loadAll("\n\n")
[ null ]
> require('js-yaml').loadAll("# hmm")
[ null ]

So load returns null if there is one document and undefined if input stream doesn't contain any.

Seems to be correct as per explanation in https://github.com/nodeca/js-yaml/issues/565#issuecomment-659696047.