mongoc_collection_create_bulk_operation_with_opts always calls mongoc_bulk_operation_set_let. If the let option was never specified, this is redundant as mongoc_bulk_operation_new already initializes it to an empty BSON document. Instead, we should gate the setter with a bson_empty check:

if (!bson_empty (&bulk_opts.let)) {
   mongoc_bulk_operation_set_let (bulk, &bulk_opts.let);
}