I would actually want to use the date instead of the aux_date whenever the aux_date is missing.
I found a (quite ugly) workaround that achieves this: Instead of using
-l "aux_date > [2020/12/19] and aux_date < [2020/12/22]"
it seems to work to use
-l "(to_string(aux_date) > to_string([2020/12/19]) and to_string(aux_date) < to_string([2020/12/22])) or (to_string(aux_date) == '' and date > [2020/12/19] and date < [2020/12/22])
which is quite ugly, but as I am abstracting via scripts anyway, this should be fine for now. If anyone has a more elegant solution, please let me know :)
Thanks,
Shredder