[RQL] add NULLS LAST and NULLS FIRST to RQL

The NULLS FIRST and NULLS LAST options can be used to determine whether nulls appear before or after non-null values in the sort ordering. By default, null values sort as if larger than any non-null value; that is, NULLS FIRST is the default for DESC order, and NULLS LAST otherwise.

As NULLS FIRST and NULLS LAST are specific features of postgres, we'll use "ORDER BY X is NULL, X" and "ORDER BY X is NOT NULL, X" for other dbms.