Add example for List.wrap(nil)

This better covers the cases of List.wrap

lib/koans/07_lists.ex

@@ -63,6 +63,10 @@ defmodule Lists do
     assert List.wrap("value") == ___
   end
 
+  koan "Wrapping nothing produces a list of nothing" do
+    assert List.wrap(nil) == ___
+  end
+
   koan "When there is already a list do not wrap it again" do
     assert List.wrap(["value"]) == ___
   end

test/koans/lists_koans_test.exs

@@ -18,6 +18,7 @@ defmodule ListsTests do
                [1, 2, 3, 4],
                {1, 2, 3},
                ["value"],
+               [],
                ["value"],
                ]