do not (try to) build hash table on void-NIL column
do not (try to) build hash table on void-NIL column

it does not make much sense to build a hash table
on a void-NIL column, and our code most probably
is not really capable of ding so correctly;
hence, we better do not even try to do so

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