Check for function first, support for Promise.all

[tprovick]

This was one of multiple promises I was waiting for via Promise.all, but if I created the promise by calling then() without passing in a function, it threw an error. It was relatively easy just to pass an empty function in, but I thought I would also add the check to the library so I didn't need to.

I thought if I found it useful, someone else might, so I created a PR.

[fregante]

You don't need to create a new promise by calling .then specifically, all tweens are already promises. Use:

Promise.all([
    TweenLite.to(...)
]);

Promise.all will pass a resolve function to gsap-then's .then so this should already work as is.

---

I can't merge this as is because it changes the return value of that .then

Sadly I didn't write any tests to verify the behavior.

[tprovick]

You're completely right. I was trying to wrap something up and didn't think this all the way through.

I was having a problem with Promise.all and a timeline, but I'll revisit and see if it can't be solved in another way. Thanks for the response.