demo: Fix crash when there are no supported devices

Handle the empty list output from fp_discover_devs() when there are no
supported devices.

Closes: #146
2 jobs for <span class="ref-name">wip/hadess/gtk-demo-crash</span> in 9 minutes and 33 seconds (queued for 1 minute and 8 seconds)
Status Job ID Name Coverage
  Build
passed #86134
build

00:03:41

 
  Test
passed #86135
flatpak master

00:05:52