Lobby: Dashboard - add a safety fence in productInfoFeedAction() in case the firewall has no intrenet access.

Fixes:

Error: Call to a member function children() on null in /usr/local/opnsense/mvc/app/controllers/OPNsense/Core/Api/DashboardController.php:201
Stack trace:

src/opnsense/mvc/app/controllers/OPNsense/Core/Api/DashboardController.php

@@ -201,6 +201,9 @@ class DashboardController extends ApiControllerBase
         $output = curl_exec($ch);
         curl_close($ch);
         $payload = simplexml_load_string($output);
+        if (empty($payload)) {
+            return $result;
+        }
         foreach ($payload->channel->children() as $key => $node) {
             if ($key == 'item') {
                 $result['items'][] = [