Simplified Common Lisp reference
Symbol class: Conses, Lists and related functions
Symbol type: function
nullobject => T or NIL
Argument description:
object an object

NULL function returns true if the argument is NIL, otherwise it returns false. NULL is identical to NOT, but used in conjunction with list processing unlike NOT which is used in boolean logic processing.

(null '()) => T
(null '(1 2 3)) => NIL
(null nil) => T
(null t) => NIL
(null 234.4) => NIL
(null "lisp") => NIL
Function indexFull documentation for null (HyperSpec)