fix(system-backup): handle snapshot of last backup removed (backport #3478)

[mergify]

Which issue(s) this PR fixes:

Issue # longhorn/longhorn#10215

What this PR does / why we need it:

The snapshot of the last backup of the volume might be removed. Therefore, volume backup will not be up to date and needs to create a new backup.

Special notes for your reviewer:

Additional documentation or context

https://ci.longhorn.io/job/private/job/longhorn-tests-regression/8300/ - passed

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This is an automatic backport of pull request #3478 done by Mergify.

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