In CDRIVER-880 we opted to hold true to our docs and return a valid cursor that would then result in failure once iterated over.

However, mongoc_collection_aggregate() can return a NULL when the server preselection fails, and no error will be reported at all as we discard of the error from the preselect:

https://github.com/mongodb/mongo-c-driver/blob/master/src/mongoc/mongoc-collection.c#L243-L251