protected function buildBody()
    {
        $message = [
            'to' => $this->getTo(),
            'registration_ids' => $this->getRegistrationIds(),
            'notification' => $this->getNotification(),
            'data' => $this->getData(),
        ];

        $message = array_merge($message, $this->getOptions());

        // remove null entries
        return array_filter($message);
    }