libudev: accept NULL as the argument 'struct udev*' for udev_monitor_new() or friends

As udev_monitor struct or friends are now almost independent of udev
struct. So, generating these objects without udev struct is reasonable.

src/shared/udev-util.c

@@ -42,7 +42,6 @@ int udev_device_new_from_stat_rdev(struct udev *udev, const struct stat *st, str
         struct udev_device *nd;
         char type;
 
-        assert(udev);
         assert(st);
         assert(ret);