How to get a customer's ID from a webhook with Braintree?

How can I get a customer's ID from a webhook notification from Braintree?

For example, I can do the following to get their ID:

gateway.webhookNotification.parse(sampleNotification.bt_signature, sampleNotification.bt_payload, (err, webhookNotification) => {
     let customerId = webhookNotification.subject.subscription.transactions[0].customer.id;
});

But this requires the user to have at least one transaction first. How can I get a customer's ID if they do not yet have any transactions?

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