Maybe this is fixed in 3.5.2 — but I doubt it. This bug has been around for years.
Periodically Aperture will show a project as empty on opening — event though an image count is displayed and images scroll if you mouse over the project. It’s a dangerous bug, with some configurations it would be easy to accidentally delete an image filled project.
The usual fix is to restart Aperture.
The only other fix I’ve found is to create a truly empty project, then drag the apparently empty project into it. All the images reappear.