photos-tracker-queue: Keep object alive when init fails

If init succeeds, we will have one singleton as before.
If init fails, we will also have one singleton, which will
return an error any time g_initable_init() is called
(thanks to 51b22906).

Previously, if init failed, the weak reference would cause the `static
GObject *self` value inside the constructor function to return to NULL,
meaning the next call to photos_tracker_queue_dup_singleton() would try
to initialise Tracker again. In some cases this could result in a very
slow startup.
1 job for sam/tracker3 in 9 minutes and 52 seconds
Status Job ID Name Coverage
failed #820979


Name Stage Failure
flatpak Test
Running after_script
Uploading artifacts for failed job
Uploading artifacts...
org.gnome.Photos.Devel.flatpak: found 1 matching files

repo.tar: found 1 matching files

.flatpak-builder/build/gnome-photos/_flatpak_build/meson-logs/meson-log.txt: found 1 matching files

.flatpak-builder/build/gnome-photos/_flatpak_build/meson-logs/testlog.txt: found 1 matching files

Uploading artifacts to coordinator... ok
id=820979 responseStatus=201 Created token=P_W3Gpwb
ERROR: Job failed: exit code 1