Work around a bug in MonetDB: the kunion() produces a bat with two nil values in the head but also with key property set.
Work around a bug in MonetDB: the kunion() produces a bat with two nil
values in the head but also with key property set.

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