use early returns in EventDispatcher@trigger

flight/core/EventDispatcher.php

@@ -34,7 +34,10 @@ class EventDispatcher
     {
         $listenerReturnValue = null;
 
-        if (isset($this->listeners[$event])) {
+        if (!isset($this->listeners[$event])) {
+            return null;
+        }
+
         foreach ($this->listeners[$event] as $listener) {
             $listenerReturnValue = $listener(...$args);
 
@@ -42,7 +45,6 @@ class EventDispatcher
                 break;
             }
         }
-        }
 
         return $listenerReturnValue;
     }