wizard-downloads: Check for OSes without medias

This is a very corner case, but it might happen that the user
does not have data in the osinfo-db at all, causing us to
populate the recommended list model with a null pointer.

Fixes #283
4 jobs for check-for-oses-without-medias-3-30 in 60 minutes and 57 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #155026
flatpak

00:45:46

 
  Review
passed #155027
review

00:01:39

manual #155028
allowed to fail manual
stop_review
 
  Cross Distro
passed #155029
fedora:rawhide

00:13:31