Hey @NK10, at the moment there's not a specific webhook subscription that would only fire when a cart is empty, and in terms of having the checkouts/updated webhook fire only when the checkout is empty is also something that isn't currently implemented. I'm not sure how you're building your integration, but if you're using our Node Library, you could use something like Express as your web framework to set up a webhook listening service that can parse the calls you're looking for so that you can build an app logic that only triggers when your app receives a cart/update webhook with no items in it.
There's a good blog post here from Bearer that goes over some of the steps on how to get started with building a webhook server with Node/Express if you wanted to check it out.
That said, I get if your intent is to lower the amount of webhook calls your getting overall though. Could you provide a use-case for when you'd want to have a webhook trigger when there's an empty cart? I can think of a few examples of why you might need it, but I'd be happy to forward on some feedback from you to our product team based on your specific use-case. I can't guarantee anything on my end, but I'm happy to pass that along - definitely would be a useful hook to have.