chore: add module concatenation hint (#2503)

Allow Webpack to process this file as an ES Module, which allows
concatenating using ModuleConcatenationPlugin, resulting in a slightly
smaller bundle and removes the overhead of a runtime `require()`.

The export default null will get optimized away during tree-shaking.

packages/@vue/cli-service/lib/commands/build/setPublicPath.js

@@ -6,3 +6,6 @@ if (typeof window !== 'undefined') {
     __webpack_public_path__ = i[1] // eslint-disable-line
   }
 }
+
+// Indicate to webpack that this file can be concatenated
+export default null