feat: make native road pipeline the default

scripts/lib/cesium-preview.js

@@ -88,6 +88,7 @@
   // The route file is an extra on top of the scene, not a precondition for it.
   // A missing or unreadable route costs the cruise controls, not the preview.
   async function fetchOptionalJson(url) {
+    if (!url) return null;
     try {
       return await fetchJson(url);
     } catch (error) {