fix(kafka sink): send no key when key_field is null - #27145

Summary

With key_field set, an event whose key field is explicitly null was produced with the literal string <null> as its Kafka key, because get_key coerced the value to bytes whatever its type. An event without the field was already produced with no key.

get_key now treats a null value like a missing one for log and trace events. Metric tags need no change: MetricTags::get already skips null values.

References

Issue: #24728

Vector configuration

sinks:
  kafka_out:
    type: kafka
    inputs: [decode_message]
    bootstrap_servers: localhost:9092
    topic: test
    key_field: key
    encoding:
      codec: json

Input {"test": "test", "key": null} now produces a message with no key, like {"test": "test"}.

How did you test this PR?

Added sinks::kafka::request_builder::tests::kafka_get_key_null_is_no_key, which checks that a null key gives no key for log and trace events and that a string key is unchanged. It fails without the change (Some(b"<null>")). The Kafka integration tests were not run.

cargo test --no-default-features --features sinks-kafka --lib sinks::kafka::request_builder

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
  For a new deprecation announcement, follow the deprecation policy and add a deprecation.d/ notice instead.
• No. A maintainer will apply the no-changelog label to this PR.

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This PR was written with Claude Opus 5.5 as a coding agent and reviewed with Codex.

Closes: #24728