40ignition-ostree: skip udev hack if Ignition did not reprovision rootfs

Currently, this code only executes via Ignition reprovisioning the
rootfs, but we're about to add code to reprovision the rootfs outside of
that path. In that case, we don't need to query the Ignition config.

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/ignition-ostree-transposefs.sh

@@ -96,8 +96,10 @@ mount_and_restore_filesystem_by_label() {
     local mountpoint=$1; shift
     local saved_fs=$1; shift
     local new_dev
-    new_dev=$(jq -r "$(query_fslabel "${label}") | .[0].device" "${ignition_cfg}")
-    udev_trigger_on_label_mismatch "${label}" "${new_dev}"
+    new_dev=$(jq -r "$(query_fslabel "${label}") | .[0].device // \"\"" "${ignition_cfg}")
+    if [ -n "${new_dev}" ]; then
+        udev_trigger_on_label_mismatch "${label}" "${new_dev}"
+    fi
     mount_verbose "/dev/disk/by-label/${label}" "${mountpoint}" rw
     find "${saved_fs}" -mindepth 1 -maxdepth 1 -exec mv -t "${mountpoint}" {} +
 }