Fix Symfony deprecation error in TriggerEventController (#1147)

* Fix Symfony deprecation error in TriggerEventController

* Remove optional in TriggerEventController

Author: pajkho

src/HttpApi/Controllers/TriggerEventController.php

@@ -16,7 +16,7 @@ public function __invoke(Request $request)
         if ($payload->has('channel')) {
             $channels = [$payload->get('channel')];
         } else {
-            $channels = $payload->get('channels', []);
+            $channels = $payload->all()['channels'] ?? [];
 
             if (is_string($channels)) {
                 $channels = [$channels];