Major - P3 Create a bulk operation with writeConcern w: 0 and run mongoc_bulk_operation_execute. The reply is:
{ "nInserted" : 0, "nMatched" : 0, "nModified" : 0, "nRemoved" : 0, "nUpserted" : 0, "writeErrors" : [ ] }
... regardless of what operations were executed. Instead, return the empty BSON "{}".
This bugfix is also a minor API change, so highlight it in the 1.4.0 release notes, and do not backport.