This removes two g_critical() errors that are seen when running the
tests in CI.

Before:

    (tracker-miner-fs:7050): libupower-glib-WARNING **: 14:31:50.109: Couldn't connect to proxy: Could not connect: No such file or directory

    (tracker-miner-fs:7050): GLib-GObject-CRITICAL **: 14:31:50.109: g_object_unref: assertion 'G_IS_OBJECT (object)' failed

    (tracker-miner-fs:7050): GLib-GObject-CRITICAL **: 14:31:50.109: g_object_unref: assertion 'G_IS_OBJECT (object)' failed

    ** (tracker-miner-fs:7050): WARNING **: 14:31:50.109: Unable to connect to UPower

After:

    (tracker-miner-fs:12620): libupower-glib-WARNING **: 14:38:52.337: Couldn't connect to proxy: Could not connect: No such file or directory

    ** (tracker-miner-fs:12620): WARNING **: 14:38:52.337: Unable to connect to UPower

Fixes GNOME/tracker-miners#75