[services] Backup - mind empty attachment holders

Summary:
Depends on D3583

Use assignItemFromDatabaseUserIDCreatedIndex method introduced in the previous diff in the DatabaseManager.

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/D3585