Fix all remaining print preview closure compiler errors

chrome/browser/resources/print_preview/data/destination_store.js

Index: chrome/browser/resources/print_preview/data/destination_store.js
diff --git a/chrome/browser/resources/print_preview/data/destination_store.js b/chrome/browser/resources/print_preview/data/destination_store.js
index bc55356e0587f8fe4b12b2ac9bb41e7fde933459..72bd0b006f13ccb1988147f1a52a724640a4a6d0 100644
--- a/chrome/browser/resources/print_preview/data/destination_store.js
+++ b/chrome/browser/resources/print_preview/data/destination_store.js
@@ -531,8 +531,8 @@ cr.define('print_preview', function() {
     __proto__: cr.EventTarget.prototype,
 
     /**
-     * @param {string=} opt_account Account to filter destinations by. When
-     *     omitted, all destinations are returned.
+     * @param {?string=} opt_account Account to filter destinations by. When
+     *     null or omitted, all destinations are returned.
      * @return {!Array<!print_preview.Destination>} List of destinations
      *     accessible by the {@code account}.
      */