allocs rpc: fix client panic on nil allocs

[gulducat]

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.

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Changes to Security Controls

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

[tgross]

LGTM!

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.

Well-worded 👍