fix(backend): handle undefined special_imap_folders in user_settings

Author: IrAlfred

modules/core/output_modules.php

@@ -616,7 +616,7 @@ protected function output() {
         $settings = $this->get('user_settings', array());
         $enable_snooze = $settings['enable_snooze'] ?? DEFAULT_ENABLE_SNOOZE;
         $enable_collect_address_on_send = $settings['enable_collect_address_on_send_setting'] ?? DEFAULT_ENABLE_COLLECT_ADDRESS_ON_SEND;
-        $specialFolders = $settings['special_imap_folders'];
+        $specialFolders = $settings['special_imap_folders'] ?? array();
         $formattedSpecialFolders = [];
         foreach ($specialFolders as $serverId => $folders) {
             $formattedSpecialFolders[$serverId] = [];