JS: support noop parentheses in js/useless-assignment-to-local

The syntatic recognizer `isNullOrUndef` did not handle expressions
that were wrapped in parentheses.

This eliminates some results here:
https://lgtm.com/projects/g/vuejs/vue/alerts?mode=tree&ruleFocus=7900088

javascript/ql/test/query-tests/Declarations/DeadStoreOfLocal/tst.js

@@ -141,4 +141,15 @@ function v() {
   x = 23;
   x = 42;
   return x;
-}
+}
+
+(function(){
+	var x = (void 0);
+	var y = ((void 0));
+	var z1 = z2 = (void 0);
+	x = 42;
+	y = 42;
+	z1 = 42;
+	z2 = 42;
+	return x + y + z1 + z2;
+});