[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.
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