Avoid warning for empty files too small to fingerprint

[an-cor]

Summary

This PR fixes #1065 by avoiding the Currently ignoring file too small to fingerprint warning for empty files.

The existing behavior emits the same warning for empty files and for non-empty files that are too small to fingerprint. Empty files can be common in production logging environments, especially with short-lived jobs or pods, so warning on them can create noisy logs without giving users actionable information.

This change keeps the existing warning behavior for non-empty files that are too small to fingerprint, but skips the warning for empty files.

Vector configuration

No full Vector configuration was used for this change. The behavior is covered at the file-source-common fingerprinter level with a targeted unit test.

How did you test this PR?

Tested locally on macOS with targeted file-source-common tests.

Commands run:

cargo test -p file-source-common no_error_on_empty_file
cargo test -p file-source-common fingerprinter
cargo fmt --check
git diff --check

Results:

• no_error_on_empty_file passed
• fingerprinter test filter passed: 8 passed, 0 failed
• cargo fmt --check passed
• git diff --check passed

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our [guidelines](https://github.com/vectordotdev/vector/blob/master/changelog.d/README.md).
• No. A maintainer will apply the no-changelog label to this PR.

References

• Closes: Only warn about small files when they are not empty #1065

Notes

This PR includes a changelog fragment:

changelog.d/1065-empty-file-fingerprint-warning.fix.md

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[datadog-vectordotdev]

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[chatgpt-codex-connector]

Keep remove_after tracking for empty files

When the file source uses checksum fingerprinting with remove_after_secs, this branch now suppresses both the warning and the insertion into known_small_files for zero-byte files. The cleanup path deletes only entries from known_small_files after the grace period (lib/file-source/src/file_server.rs cleanup loop), so an empty matched file is no longer removed even though it is already at EOF and previously was tracked for removal. Suppress emit_file_checksum_failed separately while still recording the path for remove_after_secs cleanup.

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