Fix mistake in switch-case generation of null (bug#49746)

Reported by Gregor Zattler.

* lisp/emacs-lisp/bytecomp.el (byte-compile--cond-switch-prefix):
Be more careful in the selection of equality.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test case.

test/lisp/emacs-lisp/bytecomp-tests.el

@@ -503,6 +503,12 @@
                  (:success 'good))
                (1+ x))))
       (funcall f 3))
+
+    ;; Check `not' in cond switch (bug#49746).
+    (mapcar (lambda (x) (cond ((equal x "a") 1)
+                              ((member x '("b" "c")) 2)
+                              ((not x) 3)))
+            '("a" "b" "c" "d" nil))
     )
   "List of expressions for cross-testing interpreted and compiled code.")