refactor(empty.ts): typo (angular#8398)

emppty -> empty

Author: PatrickJS

modules/empty.ts

@@ -1,4 +1,4 @@
 // This file intentionally left blank. It is used to load nothing in some cases.
 // Such as parse5/index is redirected here instead of loading into browser.
 
-export var __emppty__;
+export var __empty__;