A ship without guests is not the current plan....however, everyone above is right. If/when such a thing does happen, everyone cleans.
We sailed shortly after the time when the sailing was cancelled for a week due to the Norwalk virus. You could really tell a difference in the cleanliness of the cabins. They do a great job of keeping up with things, but she was looking a little in need of that deep cleaning on our previous sailing. After the dry dock, things really looked great in the public areas! Funny....after 10 cruises, you start to critique the maintenance!