common-lisp: (list? nil) should be false (add test)

In common-lisp, self-hosted test4 was failing and it appeared that nil
and empty list () were being conflated. The root cause is that in
common-lisp the form `(list? nil)` was returning true. Fix this in the
core `list?` function.

There was no test for this case so add one to step4 tests.

impls/common-lisp/src/core.lisp

@@ -87,7 +87,7 @@
   (make-mal-list values))
 
 (defmal list? (value)
-  (wrap-boolean (or (mal-nil-p value) (mal-list-p value))))
+  (wrap-boolean (mal-list-p value)))
 
 (defmal empty? (value)
   (wrap-boolean (zerop (length (mal-data-value value)))))

impls/tests/step4_if_fn_do.mal

@@ -6,6 +6,8 @@
 ;=>()
 (list? (list))
 ;=>true
+(list? nil)
+;=>false
 (empty? (list))
 ;=>true
 (empty? (list 1))