Fail if could not retrieve latest GH Runner version

action.sh

@@ -241,6 +241,10 @@ function start_vm {
       latest_ver=$(curl -sL https://api.github.com/repos/actions/runner/releases/latest | jq -r '.tag_name' | sed -e 's/^v//')
       runner_ver="$latest_ver"
       echo "✅ runner_ver=latest is specified. v$latest_ver is detected as the latest version."
+      if [[ -z "$latest_ver" || "null" == "$latest_ver" ]]; then
+        echo "❌ could not retrieve the latest version of a runner"
+        exit 2
+      fi
     fi
     echo "✅ Startup script will install GitHub Actions v$runner_ver"
     if $arm ; then