empty find_missing_blobs can return immediately

[palfrey]

Description

Now that #2188 is in to provide debug, I'm seeing a lot of this sort of message in Chromium builds

{"timestamp":"2026-03-12T11:35:11.157097Z","level":"DEBUG","fields":{"message":"ConnectionManager: request running","reason":"find_missing_blobs: []"},"target":"nativelink_util::connection_manager","span":{"name":"connection_manager_worker_spawn"},"spans":[{"name":"connection_manager_worker_spawn"}]}

This is an empty find_missing_blobs. Now, on the one hand, this should be pretty fast, but I'm seeing the gRPC flow clogged up with 100s of these waiting to be serviced and the overall time on that is non-trivial because of the request overhead. It's a bit of a weird request, but it's a case we should handle on a fast path.

This PR explicitly makes the gRPC find_missing_blobs return immediately for empty requests, and also implements that in general for StoreLike as well in has_many and has_with_results for the empty cases.

Type of change

Please delete options that aren't relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

bazel test //... and some Chromium build experiments

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

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This change is 

[palfrey]

/build-image nativelink-worker-init

[palfrey]

/build-image

[github-actions]

Image built and pushed!

ghcr.io/TraceMachina/nativelink-worker-init:c2832dd

[github-actions]

Image built and pushed!

ghcr.io/TraceMachina/nativelink:c2832dd