[backup-client] avoid adding attachments when uploading User Keys

Summary:
ENG-9656.

On backup service, we parse multipart in sequence, so we're going to read attachments and as a result set siwe_backup_msg to None even if this is present.

The alternative is to completely refactor how we parse Multipart on the service side - but this is quick and clean, as attachments are present only if we have User Data.

Test Plan: Run commtest

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13928