fix(backup): prevent task creation for empty directories

[b-Nollet]

Description

See zammad#15136

Prevents task creation for empty directories during mirror backup, and adds a default string for VMs that are no longer present but still have backups (instead of "Unknown VM").

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
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  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
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[b-Nollet]

What's the reference for this PR?

The change of the unknown VM label appears unrelated to preventing task creation for empty directories. Also, this new label completely bypasses the unknown VM handling done by the Vm component.

The Vm component should display task.data.name_label if the VM is unknown, if it does not, it means there is another issue.

The reference is https://help.vates.tech/#ticket/zoom/15136

We chose to add this message to make backup reports more clear (which is the goal of both this message and preventing unnecessary task creation). When a VM is deleted but its saves are still being replicated with mirror backups, the message we added is less confusing than 'Unkwown VM'.

This use case might not be so common, so maybe you think it is better to avoid bypassing the generic message ?