I am not sure why they say that but I too have heard that an empty house takes longer to sell. I bought my house empty and liked it a lot more than the homes that were furnished using someone elses tastes. It was easier to picture my furniture in the empty house IMO.
You may have to pay a small additional premium, but better than the alternative.
It was not empty but very clean and taken care of. The first house we tried to buy was empty and there was a bidding war!
I laid plywood over some milk crates, and topped with a comforter... people did not even realize they were not real! 