When running a $vectorSearch query but the index is not ready, then there is not results.

Check class metadata before running the vector search query.

In order to help the developer. When there is 0 results, check if the index exist and is ready and throw an exception if the index is missing.

https://www.mongodb.com/docs/atlas/atlas-vector-search/vector-search-stage/#fields 

Atlas Vector Search doesn't return results if you misspell the index name or if the specified index doesn't already exist on the cluster.