Empty slice generates NOT IN(NULL) SQL statement

Follow up to #61 

I have hit an issue with the decided upon solution for #61.

When the query is built as ```WHERE id IN (NULL)``` I get 0 rows as I would expect, but if the query is ```WHERE id NOT IN (NULL)``` I also get 0 rows when I would instead expect it to find every row. 

So in the case where the operation is a NOT IN it would make sense to drop the condition completely.