Sum of all NA and empty should be 0
And the `prod` of those should be 1.

xref: https://github.com/pandas-dev/pandas/issues/9422, https://github.com/pandas-dev/pandas/pull/17630, https://mail.python.org/pipermail/pandas-dev/2017-November/000657.html

We need to implement that and design and implement the alternative (either a new method, or a keyword-argument to sum and prod).