lint: Replace 'return undefined;' with 'return;'.

Also adds a custom rule to eslint. Since the recommended way of extending
eslint is to create plugins as standalone npm packages, the separate rule
is published as 'eslint-plugins-empty-returns'.

Fixes #8669.

static/js/localstorage.js

@@ -6,7 +6,7 @@ var ls = {
         try {
             return JSON.parse(str);
         } catch (err) {
-            return undefined;
+            return;
         }
     },