adds the postgres nulls support

[yab94]

Hi,

I made this PR to add a postgresql specific fonctionnality on ORDER BY clauses :

POSTGRESQL DOC :
https://www.postgresql.org/docs/8.3/queries-order.html

It allows ORDER BY clauses like :

SELECT field FROM table ORDER BY field ASC NULLS FIRST;

This commit adds to the "QuerySort" type a new optional "nulls" props and concat it to the returned SQL clause.

I set the "nulls" props type to string but it could probably be a fixed ENUM with NULLS FIRST | NULLS LAST.