[FIX] Certificate: Allow thumbnail without background image

Hashing used the background RID for the thumbnail, so saving
a thumbnail without a background image called serialize() on null.

Author: mjansenDatabay

components/ILIAS/Certificate/classes/class.ilCertificateGUI.php

@@ -417,7 +417,7 @@ private function saveCertificate(ilPropertyFormGUI $form, array $form_fields, in
                         $xslfo,
                         isset($new_background_rid) ? $new_background_rid->serialize() : '',
                         $jsonEncodedTemplateValues,
-                        isset($new_tile_rid) ? $new_background_rid->serialize() : ''
+                        isset($new_tile_rid) ? $new_tile_rid->serialize() : ''
                     ])
                 );