Fix a crash on group_by of empty list. Fixes #208.

tests/all.test

@@ -610,6 +610,10 @@ unique
 [1,2,5,3,5,3,1,3]
 [1,2,3,5]
 
+unique
+[]
+[]
+
 [min, max, min_by(.[1]), max_by(.[1]), min_by(.[2]), max_by(.[2])]
 [[4,2,"a"],[3,1,"a"],[2,4,"a"],[1,3,"a"]]
 [[1,3,"a"],[4,2,"a"],[3,1,"a"],[2,4,"a"],[4,2,"a"],[1,3,"a"]]