various: use id128_from_string_not_null()

No functional change. In config_parse_address_generation_type() we would set
the output parameter and then say it's ignored, so it _looked_ like an error in
the code, but the variable was always initialized to SD_ID128_NULL anyway, so
the code was actually fine.

src/nspawn/nspawn.c

@@ -976,13 +976,12 @@ static int parse_argv(int argc, char *argv[]) {
                         break;
 
                 case ARG_UUID:
-                        r = sd_id128_from_string(optarg, &arg_uuid);
-                        if (r < 0)
-                                return log_error_errno(r, "Invalid UUID: %s", optarg);
-
-                        if (sd_id128_is_null(arg_uuid))
+                        r = id128_from_string_nonzero(optarg, &arg_uuid);
+                        if (r == -ENXIO)
                                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                                        "Machine UUID may not be all zeroes.");
+                        if (r < 0)
+                                return log_error_errno(r, "Invalid UUID: %s", optarg);
 
                         arg_settings_mask |= SETTING_MACHINE_ID;
                         break;