added deserialization tests

src/main/java/mops/gruppen2/domain/event/DeleteUserEvent.java

@@ -11,4 +11,6 @@ public class DeleteUserEvent extends Event {
     public DeleteUserEvent(Long event_id, Long group_id, String user_id) {
         super(event_id, group_id, user_id);
     }
+
+    public DeleteUserEvent() {}
 }