list.contains(object) returns null instead of false when list is assigned to null

list.contains(object) returns null instead of false when list is assigned to null

Consider the following code:
  1. list1 = list();
  2. list1 = null;
  3. info list1.contains(1);

Info returns:
null
(see attached screenshot)

According to the documentation of List.contains(), the only possible return values of List.contains() are true and false. But in the code above, it returns null.