Backport of scheduler(system): Fix potential panic in deployment handling. into release/1.11.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #27571 to be assessed for backporting due to the inclusion of the label backport/1.11.x.

The below text is copied from the body of the original PR.

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When a system job deployment is successful and a task group has no feasible candidate nodes, the task group's deployment state is set to nil in the mapping. If the deployment is persisted to state, because the job has multiple task groups and at least one results in successful placements, a subsequent evaluation likely triggered by a recovering node will look up the previous deployment state. It was at this point, the scheduler was not correctly handling the nil object.

Another option could be to ensure the state write never includes a nil deployment object for a task group. It would require more work in upsert handling, so I feel this is the right approach as it's defensive and will always work.

Links

Jira: https://hashicorp.atlassian.net/browse/NMD-1266
Closes: #27567

Contributor Checklist

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  changelog entry using the make cl command.
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  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
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  Please also consider whether the change requires notes within the upgrade
  guide. If you would like help with the docs, tag the nomad-docs team in this PR.

Reviewer Checklist

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Overview of commits

• f953716