Fix that projects without content are served for all platforms

[tanius]

The Graphryder 2 frontpage contains a dropdown to choose a platform and then another dropdown to choose an ethnographic project hosted on that platform.

That second platform is supposed to only list projects found on the platform chosen with the first dropdown. However, projects that contain no content (no codes, no annotations) will always be listed in the "Project" dropdown, independent of which platform has been chosen in the "Platform" dropdown or to which platform that project actually belongs.

A simple solution could be to not show projects without content, at all. There is nothing to see there anyway.