[backup-service] make user_data and user_keys optional in BackupData

Summary:
Part of ENG-9673.

Tracked in ENG-9649.

Depends on D13877

Test Plan: Tested in D13879

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13878