Backrest Max Unused doesn't work when set to 0

[sikhness]

When you set the prune policy max unused to 0%, it defaults to the 5% in the prune logs. This might be happening because when you set it to 0% in the UI, it removes the --max-unused parameter entirely in the restic command as per the logs instead of explicitly setting it to --max-unused 0.

I'm using the latest version (1.8.0).