Support empty files, "literal" nil, literal false

Stumbled upon the empty file issue, but I was treating it as an
Enumerable, anyhow, so this wouldn't have really "solved" anything. But
at least it won't raise an error that's obviously a lie.

I really have no idea why anyone would want top-level keys that only
return `nil` or `false`, either, but I'll just add support for it since
I'm already here.
v1.0.0

After years in production. Probably about time!