So far we've been implicitly relying on this by the user passing search
terms that filter out the opening balance transaction. That will stop
happening with the aging report, so we need to do it ourselves.
This makes the output more useful for broad searches like on an
entity. Invoices that cross FY boundaries will appear to be paid
without being accrued, and so would appear when we were just
filtering zeroed-out invoices.
If we integrate the aging report into this module in the future,
that'll need to follow different logic, and just filter out
zeroed-out invoices. But the basic balance report and outgoing
report are more workaday tools, where more filtering makes them
more useful.
