Only set requests in tracking request set when correct value is given

refs matomo-org/plugin-QueuedTracking#153 (comment)

Possible warning:

>  /usr/share/piwik/core/Tracker/RequestSet.php(36): Warning - Invalid argument supplied for foreach() -

Not sure how this would happen and it might hide an actual error maybe. I've been looking through code for a while and couldn't really see where this might happen.  The only one I could find was bulk tracking a wrong JSON being set. Maybe we should throw an invalid tracking parameter exception when `requests` is not an array? https://github.com/matomo-org/matomo/blob/4.4.1/plugins/BulkTracking/Tracker/Requests.php#L78

core/Tracker/RequestSet.php

@@ -33,6 +33,10 @@ public function setRequests($requests)
     {
         $this->requests = array();
 
+        if (empty($requests)|| !is_array($requests)) {
+            return;
+        }
+
         foreach ($requests as $request) {
             if (empty($request) && !is_array($request)) {
                 continue;