network: Show "no VPN configured" placeholder when last VPN removed

Fixes #765
11 jobs for network-remove-vpn-placeholder in 4 minutes and 32 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build #513934

00:01:46

 
  Test
passed asan #513936

00:02:45

passed test #513935

00:01:07

 
  Manual
manual armv8 #513941
allowed to fail manual
manual flatpak #513937
allowed to fail manual
manual msan #513940
allowed to fail manual
manual ppc64le #513942
allowed to fail manual
manual tsan #513938
allowed to fail manual
manual ubsan #513939
allowed to fail manual
 
  Review
manual review #513943
allowed to fail manual
manual stop_review #513944
allowed to fail manual