If there is no input to a combiner (e.g. you pass it an empty PCollection),
a default value is produced (the Combine function tells Beam what it's
default value is). Logically, the windowed-extension of this would be to
produce a default value whenever there is no data for a window. The problem
is
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 g
