Add empty locale extender to clear cache properly on enable/disable.

See https://github.com/flarum/core/pull/2020#issuecomment-616149583

extensions/lock/extend.php

@@ -31,6 +31,8 @@ return [
     (new Extend\Frontend('admin'))
         ->js(__DIR__.'/js/dist/admin.js'),
 
+    new Extend\Locales(__DIR__.'/locale'),
+
     function (Dispatcher $events) {
         $events->listen(ConfigureDiscussionGambits::class, function (ConfigureDiscussionGambits $event) {
             $event->gambits->add(LockedGambit::class);