Make isEmpty a type guard

[yokuze]

Currently, isEmpty just returns a boolean. It would be much more useful if it were a type-guard that asserted that its parameter was one of the "empty" object types its checking for.

[crgwbr]

@yokuze This is easy for null, undefined, array, and string types. But isEmpty currently also works for the IArguments type, which isn't generic. So, there is no type for empty arguments vs. non-empty arguments. So… thoughts?

[yokuze]

I don't think we need to differentiate between empty vs non-empty IArguments in the type guard assertion. It would be nifty if there were an existing way to do so, but I don't think it'll be that helpful to us to make that distinction.

By far, the most frequent pain point that we are trying to solve by making this function a type guard is something like this:

function doSomething(withThis: string | undefined): void {
   if (isEmpty(withThis)) {

   } else {
      // TypeScript should at least know here that `withThis` not `undefined`
   }
}

Anything beyond that is extra credit IMO.

That said, I may be missing something, so let me know if there's a use case you think we should support by differentiating between empty vs non-empty IArguments. If there is, the only option I can think of right now would be to extend the IArguments type like:

interface IEmptyArguments extends IArguments {
   length: 0;
}