In DBClientBase::query, if DBClientCursor can't be initialized, we return NULL. We should throw (perhaps, a UserException) instead.
Once this is done, we can get rid of _safeCursor in cluster_client_internal.h.

https://github.com/mongodb/mongo/blob/9bf7c1df8376377197f998f35566cc7e7f5ca03e/src/mongo/client/dbclient.cpp#L1007