Minor: Plugin: Add CAS logout button: Fix missing variable declaration

plugin/add_cas_logout_button/plugin.php

@@ -27,6 +27,7 @@ $form->addElement('text', 'cas_logout_comment', 'CAS logout description', '');
 $form->addElement('text', 'cas_logout_image_url', 'Logo URL if any (image, 50px height)');
 $form->addButtonSave(get_lang('Save'), 'submit_button');
 //get default value for form
+$defaults = [];
 $tab_default_add_cas_logout_button_cas_logout_label = api_get_setting('add_cas_logout_button_cas_logout_label');
 $tab_default_add_cas_logout_button_cas_logout_comment = api_get_setting('add_cas_logout_button_cas_logout_comment');
 $tab_default_add_cas_logout_button_cas_logout_image_url = api_get_setting('add_cas_logout_button_cas_logout_image_url');