dev: make groupByDateSpan return data when first DateSpan's begin is Nothing

[jneubrand]

There doesn't seem to be any report code that would hit this bug—just noticed it while working on a script :)

[simonmichael]

Thanks!

Is it a bug ? I can't quite tell. If colspans is empty, there's nothing to dropWhile anyway, so the true or false fallback has no effect. It seems like this code could be simpler.

[jneubrand]

Well, it's a bug in that the code does something unintuitive but only in a path that isn't hit by any code that can be called via the CLI (because you can't provide custom spans to any function that uses groupByDateSpan.)

The fallback doesn't apply [edit: just] when colspans is an empty list, but [edit: also] when the first span has no begin date. So when passing colspans [DateSpan ..2022-12-31,DateSpan 2023] to groupByDateSpan, all postings are discarded instead of none and [(DateSpan ..2022-12-31,[]),(DateSpan 2023,[]))] is returned.

[edit: …and in the case that colspans is an empty list, groupByCols [] _ = [] applies anyway, so behavior is unchanged by this PR]

[simonmichael]

I believe you. Thank you!