Handle cases where DataTransfer has a null +files+ slot

[kofno]

On firefox, the +files+ property on the DataTransfer object is
intentionally left null from dragover and dragenter events.

Relevant bug: https://bugzilla.mozilla.org/show_bug.cgi?id=640534

Since this isn't likely to be fixed by mozilla, the files function should
account for the possibility of a null.

[garyb]

Thanks for addressing this. Elsewhere we use Nullable rather than Maybe to handle this kind of thing in the API though, so would you mind using that here as well? You don't have to do anything in the JS for Nullable, you can just use it the result type.

[kofno]

Sure thing. I didn't notice that.

[kofno]

I think this is ready now