ORDER BY xx NULLS first/last break reverse_sql_order

Oracle and PostgreSQL supports "NULLS first/last" in order by statement.
It will break the reverse_sql_order by concating " DESC" to the order by
statement. The fix will both reverse the column order and NULLS order.

For example, "name ASC NULLS LAST" will be reversed to "name DESC NULL FIRST"

    User.order("name ASC NULLS LAST").last