[EventDispatcher] Replace deprecated implicit nullable (SonsOfPHP#240)

Author: MGatner

src/SonsOfPHP/Component/EventDispatcher/EventDispatcher.php

@@ -23,7 +23,7 @@ public function __construct(
      * @param string|null $eventName
      *   Is the event name is null, is will use the event's classname as the Event Name
      */
-    public function dispatch(object $event, string $eventName = null): object
+    public function dispatch(object $event, ?string $eventName = null): object
     {
         $eventName ??= $event::class;