Allow nil:bat as argument for candidate list in batcalc.* operations.
Allow nil:bat as argument for candidate list in batcalc.* operations.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb