remove undefined; only use null now

tests/test_module.ce

@@ -1,7 +1,7 @@
 // Test module - should NOT have access to $_
 
 function test() {
-  if (typeof $_ != 'undefined') {
+  if (typeof $_ != 'null') {
     log.error("✗ Module incorrectly has access to $_!");
     return "ERROR: Module has $_ access";
   } else {