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.
1 job for gnome-3-38 in 4 minutes and 47 seconds
Status Job ID Name Coverage
  Test
failed #1324052
flatpak
flatpak

00:04:47

 
Name Stage Failure
failed
flatpak Test
Uploading artifacts for failed job
Uploading artifacts...
WARNING: gnome-maps-git.flatpak: no matching files
WARNING: repo.tar: no matching files
WARNING: .flatpak-builder/build/gnome-maps/_flatpak_build/meson-logs/meson-log.txt: no matching files
WARNING: .flatpak-builder/build/gnome-maps/_flatpak_build/meson-logs/testlog.txt: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1