webui: add missing default value to fix possible login problems

This commit adds a missing default value to a hidden form element.

It fixes a possible login problem, which could occur when you send login
form data before fetching package update information in the background
finished with success or an error.

Special thanks goes to Alvaro Flaño who reported the issue and also has
put some effort into solving it.

webui/module/Auth/src/Auth/Form/LoginForm.php

@@ -161,7 +161,7 @@ public function __construct($config=null, $dird=null)
          'name' => 'bareos_updates',
          'type' => 'Zend\Form\Element\Hidden',
          'attributes' => array(
-            'value' => '',
+            'value' => 'false',
             'id' => 'bareos_updates'
          ),
       )