How to use the node-opcua-nodeid.NodeId.NullNodeId function in node-opcua-nodeid

assert(request.hasOwnProperty("requestedMaxKeepAliveCount"));  // Counter
    assert(request.hasOwnProperty("maxNotificationsPerPublish"));  // Counter
    assert(request.hasOwnProperty("publishingEnabled"));           // Boolean
    assert(request.hasOwnProperty("priority"));                    // Byte

    const subscription = new Subscription({
        publishingInterval: request.requestedPublishingInterval,
        lifeTimeCount: request.requestedLifetimeCount,
        maxKeepAliveCount: request.requestedMaxKeepAliveCount,
        maxNotificationsPerPublish: request.maxNotificationsPerPublish,
        publishingEnabled: request.publishingEnabled,
        priority: request.priority,
        id: _get_next_subscriptionId(),
        // -------------------
        publishEngine: session.publishEngine, //
        sessionId: NodeId.NullNodeId
    });

    // add subscriptionDiagnostics
    engine._exposeSubscriptionDiagnostics(subscription);

    assert(subscription.publishEngine === session.publishEngine);
    session.publishEngine.add_subscription(subscription);

    subscription.once("terminated",function() {
        const subscription = this;
        engine._unexposeSubscriptionDiagnostics(subscription);
    });

    return subscription;
};