[services] Backup - Handle empty attachment holders

Summary:
Depends on D3558

We have to cover the case when there are no attachments in the backup(in theory this is possible)

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, geekbrother, jimpo, varun

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3559