Avoid pushing null values into KVCachedFile cache

[ihtefyw]

Currently, querying a value from the file in the kv_cached_file could result into pushing a null value into the cache if it was not present on the file.
This PR fixes it and adds a test for it

[Copilot]

Pull request overview

This PR prevents KVCachedFile.Get from caching missing keys by returning early when the underlying KVFile.Get returns nil, and adds a test to ensure the cache remains unmodified on such misses.

Changes:

• Add an early return in KVCachedFile.Get when the backing file returns nil (unknown key).
• Extend the existing “unknown key” test to assert the cache does not contain the key after the miss.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
go/backend/kv_file/kv_cached_file.go	Avoids caching absent values by returning immediately when the backing file returns nil.
go/backend/kv_file/kv_cached_file_test.go	Adds an assertion that a miss (Get returning nil) does not populate the cache.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
go/backend/kv_file/kv_cached_file.go	85.71% <100.00%> (+0.24%)	⬆️

... and 2 files with indirect coverage changes

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