Fix default for num-nodes in ginkgo-e2e.sh

hack/ginkgo-e2e.sh

@@ -87,7 +87,6 @@ elif [[ ${GINKGO_PARALLEL} =~ ^[yY]$ ]]; then
   ginkgo_args+=("--nodes=30") # By default, set --nodes=30.
 fi
 
-
 # The --host setting is used only when providing --auth_config
 # If --kubeconfig is used, the host to use is retrieved from the .kubeconfig
 # file and the one provided with --host is ignored.
@@ -105,8 +104,8 @@ export PATH=$(dirname "${e2e_test}"):"${PATH}"
   --cluster-tag="${CLUSTER_ID:-}" \
   --repo-root="${KUBE_VERSION_ROOT}" \
   --node-instance-group="${NODE_INSTANCE_GROUP:-}" \
-  --num-nodes="${NUM_NODES:-}" \
   --prefix="${KUBE_GCE_INSTANCE_PREFIX:-e2e}" \
+  ${NUM_NODES:+"--num-nodes=${NUM_NODES}"} \
   ${E2E_CLEAN_START:+"--clean-start=true"} \
   ${E2E_MIN_STARTUP_PODS:+"--minStartupPods=${E2E_MIN_STARTUP_PODS}"} \
   ${E2E_REPORT_DIR:+"--report-dir=${E2E_REPORT_DIR}"} \