If a buyer receives an empty box, they do have to open a return.

If they start an 'item not received' case and the seller has tracking proving delivery, the case will close in favour of the seller.

If the buyer has started a return using, for example, the reason 'not as described' or 'items missing' you will be expected to refund.

For Simple Delivery protection to work in your favour, the buyer must use 'arrived damaged/damaged in transit'.

If they used any other reason, your only option is to try and contact Ebay and see if the reason can be changed.