nspawn: ignore NULL machine ID in the container

Previously, when the NULL (all zero) machine ID is configured in the
container, nspawn refused to execute.

Now id128_get_machine() is used, so NULL machine ID is refused with
-ENOMEDIUM, and fallback to specified UUID or randomly generated one.

src/nspawn/nspawn.c

@@ -2828,8 +2828,6 @@ static int mount_tunnel_open(void) {
 }
 
 static int setup_machine_id(const char *directory) {
-        const char *etc_machine_id;
-        sd_id128_t id;
         int r;
 
         /* If the UUID in the container is already set, then that's what counts, and we use. If it isn't set, and the
@@ -2839,9 +2837,7 @@ static int setup_machine_id(const char *directory) {
          * in the container and our idea of the container UUID will always be in sync (at least if PID 1 in the
          * container behaves nicely). */
 
-        etc_machine_id = prefix_roota(directory, "/etc/machine-id");
-
-        r = id128_read(etc_machine_id, ID128_FORMAT_PLAIN, &id);
+        r = id128_get_machine(directory, &arg_uuid);
         if (r < 0) {
                 if (!ERRNO_IS_MACHINE_ID_UNSET(r)) /* If the file is missing, empty, or uninitialized, we don't mind */
                         return log_error_errno(r, "Failed to read machine ID from container image: %m");
@@ -2851,12 +2847,6 @@ static int setup_machine_id(const char *directory) {
                         if (r < 0)
                                 return log_error_errno(r, "Failed to acquire randomized machine UUID: %m");
                 }
-        } else {
-                if (sd_id128_is_null(id))
-                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
-                                               "Machine ID in container image is zero, refusing.");
-
-                arg_uuid = id;
         }
 
         return 0;