lib: simplify check in child_process

PR-URL: #37367
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

lib/child_process.js

@@ -99,8 +99,7 @@ function fork(modulePath /* , args, options */) {
     args = arguments[pos++];
   }
 
-  if (pos < arguments.length &&
-      (arguments[pos] === undefined || arguments[pos] === null)) {
+  if (pos < arguments.length && arguments[pos] == null) {
     pos++;
   }