O yes, I see that now. Even a new post will have at least one group of fields, even if nothing was filled out yet. So, if processing a group, we should always check for a group of fields without a value, or with a default value.
I'm not sure if this makes sense. For example: the post is a Product, and the cloneable group is possible Discounts. Then there's always at least one discount, with empty fields, even if the Product is not discounted.
It's unexpected, but good to know it works this way 🙂