udevadm-settle: fixed return code for empty queue

If the udev queue is empty and "/run/udev/queue" does not exist,
"udevadm settle" would return with EXIT_FAILURE, because the inotify on
"/run/udev/queue" would fail with ENOENT.

This patch lets "udevadm settle" exit with EXIT_SUCCESS in this case.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
gudev: add missing (nullable) annotations on return values

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
hwdb: update

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>