Backport of allocs rpc: fix client panic on nil allocs into release/2.0.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #28187 to be assessed for backporting due to the inclusion of the label backport/2.0.x.

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

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Description

A client can panic on a nil alloc sent by the server if the alloc gets GC'd at an innoportune moment, namely between two distinct RPC calls in the same loop.

The server should never return a nil alloc. Instead, return a partial slice that is shorter than the number of allocs requested.

Fixes: #28172

Note: At this time, the client does not make use of the partial list; it simply discards the results and retries in a couple seconds (client/client.go line 2683). Potential client optimizations may be revisited as a follow-up.

Testing

Server-side unit tests ensures no nil allocs in the response. I'll explore Client-side tests, but it'll take some extra doing to set up mocks for the RPC calls.

Contributor Checklist

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• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
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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.
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Reviewer Checklist

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• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

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Overview of commits

• 84f172e