Fix division by zero in delete-disconnected-fs-submissions

[heathdutton]

Fixes #7201

When deleting disconnected filesystem submissions, if a file has zero bytes, rate becomes 0 after the first deletion. On subsequent iterations, the ETA calculation filesize / rate causes a ZeroDivisionError.

This adds a guard to skip the ETA calculation when rate is zero.

Test plan

1. Create a disconnected empty file in the store
2. Run manage.py delete-disconnected-fs-submissions --force
3. Verify no ZeroDivisionError occurs

Checklist

This change accounts for:

• any required additional documentation
• any necessary AppArmor changes (added or removed application files)
• any impact on new SecureDrop installs and upgrades
• our dependency update policy

[legoktm]

@heathdutton hi, thanks for submitting all these PRs! It'll take us a bit of time to get to reviewing them as our main priority is on the SecureDrop Workstation side of things right now. And you don't need to keep updating them, if there are conflicts we'll rebase when needed.

Also please let us know if you used any AI/LLM tools when creating these so we can review accordingly.