reduce: handle empty updates (fix #1313)

tests/jq.test

@@ -686,6 +686,11 @@ reduce [[1,2,10], [3,4,10]][] as [$i,$j] (0; . + $i * $j)
 null
 14
 
+# Test fix for #1313 (reduce should handle empty updates)
+reduce range(5) as $n ([]; select($n%2 == 1) | . + [$n])
+null
+[1,3]
+
 . as $dot|any($dot[];not)
 [1,2,3,4,true,false,1,2,3,4,5]
 true