dir: Squash empty collection IDs into NULL ones
dir: Squash empty collection IDs into NULL ones

repo_get_remote_collection_id() already squashes empty ("") collection
IDs into NULL, and ignores the collection ID when P2P support isn't
enabled. So use that helper function in
flatpak_dir_get_remote_collection_id() for consistency. At the moment
that function is only used in tests, so this doesn't immediately affect
anything.

Closes: #1411
Approved by: alexlarsson

handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak