Fix last(empty)

It's actually empty now. (Makes last/1 even slower than before, though.)

src/builtin.jq

@@ -164,7 +164,7 @@ def all(condition): all(.[]; condition);
 def any(condition): any(.[]; condition);
 def all: all(.[]; .);
 def any: any(.[]; .);
-def last(g): reduce . as $_ (.; g);
+def last(g): reduce . as $_ (.; g|[.]) | .[]?;
 def nth($n; g): if $n < 0 then error("nth doesn't support negative indices") else last(limit($n + 1; g)) end;
 def first: .[0];
 def last: .[-1];

tests/jq.test

@@ -333,6 +333,14 @@ first(1,error("foo"))
 null
 1
 
+[first(empty)]
+null
+[]
+
+[last(empty)]
+null
+[]
+
 #
 # Check that various builtins evalute all arguments where appropriate,
 # doing cartesian products where appropriate.