No staged mutations doesn't return early in txns

See https://github.com/couchbase/gocbcore/blob/master/transactionattempt_commit.go#L419 after we handle the error we do not return. Same at https://github.com/couchbase/gocbcore/blob/master/transactionattempt_commit.go#L571. This probably isn't a bug that is being hit but we should address it.