For example the dependecy chain is the following: A->B->C. When a
question (A) depends on another dependent item (B) and B hasn't
displayed (because of C's response), the $value for the B's response
will be null. In this case the can_see_item() method returned
null. Because the can_see_item() returned null (not false), the
get_pages() method displayed the question A, because it checks for
explicit false: $this->can_see_item($item) !== false.
Now, false is also returned, if the dependent question is not visible.