Fix iterating over an empty list.

test/list/iterate.wren

@@ -5,3 +5,6 @@ IO.print(a.iterate(1)) // expect: 2
 IO.print(a.iterate(2)) // expect: 3
 IO.print(a.iterate(3)) // expect: false
 IO.print(a.iterate(-1)) // expect: false
+
+// Nothing to iterate in an empty list.
+IO.print([].iterate(null)) // expect: false

test/list/plus.wren

@@ -13,3 +13,6 @@ IO.print(d) // expect: [1, 2, 3, 4, 5, 6]
 IO.print(e) // expect: [1, 2, 3]
 IO.print(f) // expect: [1, 2, 3]
 IO.print(g) // expect: []
+
+// Doesn't modify original list.
+IO.print(a) // expect: [1, 2, 3]