metadata: Prevent usage of NULL if GUdevDevice is not found

Code to determine tree name from device has been recently ported from
udev to gudev by commit c2d85644. Unfortunately, the gudev code floods
logs by the following messages if device is not found (it can happen
e.g. for tmpfs):
g_udev_device_has_property: assertion 'G_UDEV_IS_DEVICE (device)' failed

Udev code silently returned if NULL device was used, however, corresponding
gudev code prints the mentioned messages. Let's prevent usage of NULL
device in order to prevent those messages.

https://bugzilla.gnome.org/show_bug.cgi?id=795191