fix: make last(empty) yield no output values (#3179)

tests/man.test

@@ -881,10 +881,14 @@ false
 [0,1,2,3,4,5,6,7,8,9]
 [3,4,5,6,7,8,9]
 
-[first(range(.)), last(range(.)), nth(./2; range(.))]
+[first(range(.)), last(range(.)), nth(5; range(.))]
 10
 [0,9,5]
 
+[first(empty), last(empty), nth(5; empty)]
+null
+[]
+
 [range(.)]|[first, last, nth(5)]
 10
 [0,9,5]