Fix defaulting in cluster/saltbase/install.sh

cluster/saltbase/install.sh

@@ -73,7 +73,7 @@ kube_bin_dir="/srv/salt-new/salt/kube-bins";
 docker_images_sls_file="/srv/salt-new/pillar/docker-images.sls";
 for docker_file in "${KUBE_DOCKER_WRAPPED_BINARIES[@]}"; do
   docker_tag=$(cat ${kube_bin_dir}/${docker_file}.docker_tag);
-  if [[ ! -z "${KUBE_IMAGE_TAG}" ]]; then
+  if [[ ! -z "${KUBE_IMAGE_TAG:-}" ]]; then
     docker_tag="${KUBE_IMAGE_TAG}"
   fi
   sed -i "s/#${docker_file}_docker_tag_value#/${docker_tag}/" "${docker_images_sls_file}";