fix(bindCallback): emit undefined when callback is without arguments

[mpodlasin]

Description
Related to issue #2254 in bindNodeCallback. It turns out bindCallback has the same problem:

When wrapped function calls its callback without any arguments, resulting Observable will emit empty array, intead of undefined.

function callback(cb) {
   cb();
}

Observable.bindCallback(callback)
.subscribe(value => console.log(value)); // value is []

In RxJS 4 in fromCalllback resulting Observable emits undefined as seen here:
https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/perf/operators/fromcallback.js#L20-L21
(when array is empty, calling results[0] results in undefined, which is emitted).

Related to
#2254

[coveralls]

Coverage remained the same at 97.689% when pulling 915a2a8 on Podlas29:bind-callback-no-callback-arguments into 6ce4773 on ReactiveX:master.

[kwonoj]

Change looks ok to me.

[kwonoj]

It's not related to this PR, but I just noticed bindNodeCallback doesn't have corresponding overloading signatures. Maybe we need this? /cc @david-driscoll

[mpodlasin]

Now that I think about it, I have seen some operators have type-only tests. Maybe I should add such tests here?

[kwonoj]

No, that is intended for specific cases if current test case doesn't covers it. it is worth to create those and it'll be appreciated, but I'd like to suggest to separate with this PR since each has different scope.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.