Letting these statements speak for themselves:
(setq a 2 b 4 c nil d 8)
(setq lst_1 '(a b c d)) -> (A B C D)
(setq lst_2 (list a b c d)) -> (2 4 nil 8)
(null 'C) -> nil
(null (eval C)) -> T
(null (nth 2 lst_1)) = (null 'C)
(null (nth 2 lst_2)) = (null (eval C))