layersPopover: Fix crash when no aerial tiles defined

Fixes a crash when no aerial tiles is defined.
1 job for gnome-3-36 in 4 minutes and 34 seconds
Status Job ID Name Coverage
  Test
failed #1331855
flatpak
flatpak

00:04:34

 
Name Stage Failure
failed
flatpak Test
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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