Translate NULLS FIRST and NULLS LAST in ORDER BY clause

[ianmcook]

Currently queryparser will throw an error if the NULLS FIRST or NULLS LAST keywords are used in the ORDER BY clause. A future version of queryparser should recognize and parse these keywords and modify the order_by sublist accordingly. For example, the clause ORDER BY x NULLS FIRST would be translated as follows:

$order_by
$order_by[[1]]
is.na(x)

$order_by[[2]]
x