Report breaks due to resample #22
Description
Some reports (w.g. wl report --date-from 2020-10 --date-to 2020-11) fail with an error message TypeError: Only valid with DatetimeIndex, TimedeltaIndex or PeriodIndex, but got on instance of 'Index'. It seems that this is the case if one week is without a single entry, but I have to further investigate.