Often it is useful to test whether a given Scheme object is a list or not. List-processing procedures could use this information to test whether their input is valid, or they could do different things depending on the datatype of their arguments.
null? is often used in list-processing code to
tell whether a given list has run out of elements. That is, a loop
somehow deals with the elements of a list until the list satisfies
null?. Then, the algorithm terminates.
Return 1 when x is the empty list; otherwise return 0.