mapView: Don't try to set aerial tiles if not available

Safe-guard agains setting the aerial tile source
if it's not available in the service file.
This avoid a crash if aerial was saved as last-used
map type in gsettings and at next startup the service
has dropped support.
2 jobs for master in 12 minutes and 20 seconds
Status Job ID Name Coverage
  Test
passed #1324049
flatpak
flatpak

00:10:05

 
  Deploy
passed #1324050
nightly

00:02:15