cars: import handles models without years, keeps roman numerals upper-case; don't cache an empty overview

src/lib/server/cars.ts

@@ -132,7 +132,7 @@ export async function carOverview(): Promise<Overview> {
 			nlCars: Number(counts.nlCars)
 		}
 	};
-	overviewCache = { at: Date.now(), value };
+	if (value.stats.models) overviewCache = { at: Date.now(), value };
 	return value;
 }