Checkout should not be able if the subtotal is 0€

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As the title suggest, there should be an option to disable the checkout if the subtotal is 0€. We use an app which adds a free gift to the cart, after a certain amount is reached. People can remove these items and continue to checkout with the free gift, resulting in an order, which only has the free item.

The app removes it after the order went through, but i dont want an order to be put in the first place. Any suggestions?