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/libudev/libudev-queue.c

@@ -45,11 +45,6 @@ _public_ struct udev_queue *udev_queue_new(struct udev *udev)
 {
         struct udev_queue *udev_queue;
 
-        if (udev == NULL) {
-                errno = EINVAL;
-                return NULL;
-        }
-
         udev_queue = new0(struct udev_queue, 1);
         if (udev_queue == NULL) {
                 errno = ENOMEM;