Although it's too late now, the best thing to do is contact the seller first - going straight to a dispute may put them off-side, and in this particular instance, they may not fully understand the situation, depending on what kind of dispute you opened and what documentation they have. Did the seller communicate via the dispute at any point, and more importantly, what kind of dispute did you open?

Hopefully it was for Item Significantly Not As Described, because there's an option under that for receiving and empty box (this is as opposed to Item Not Received, because if the seller can provide proof that a package was sent/received, the dispute will close in their favour since the contents of the parcel isn't under dispute in a Not Received claim).