When sorting an empty array with the "sort" filter, it returns nil
instead of []. This confuses subsequent filters in the chain that
expect an array. For example, when followed by the "map" filter, it
produces an array containing one nil element: [nil].
I could special-case the nil return value, but that would be more
cumbersome than making sure "sort" always returns an array.
Add a case to the "sort" method to return [] if the array is empty,
before performing any checks on ary.first that assume a non-empty array.
There is still a danger of returning nil if the first item in the array
is nil and it is non-empty, but I'm not sure how better to handle that
case.
Apply a similar fix to sort_natural, uniq, and compact filters.
