ensure that app:siteUrl is required

src/server/form/admin/app.js

@@ -5,7 +5,7 @@ var form = require('express-form')
 
 module.exports = form(
   field('settingForm[app:title]').trim(),
-  field('settingForm[app:siteUrl]').trim().isUrl(),
+  field('settingForm[app:siteUrl]').trim().required().isUrl(),
   field('settingForm[app:confidential]'),
   field('settingForm[app:fileUpload]').trim().toBooleanStrict()
 );