I'm trying to work with windows and I was hoping my current batch workflows could work transparently with and without windows (I know "without windows" really means with GlobalWindows), but any batch workflow that uses a `CombineGlobally` doesn't work without using `without_defaults` otherwise it gets the following error
> Default values are not yet supported in CombineGlobally() if the output PCollection is not windowed by GlobalWindows.... If a batch workflow uses a composite transform that contains a `CombineGlobally`, I think there is no solution. What exactly are these combiner defaults? Can `.without_defaults` be the "default" setting of CombineGlobally? It's unfortunate to have to consider the non-global window case explicitly and add these `.without_defaults()`
