kafka sink: Fix: Correctly write out negative timestamps with Avro

[def-]

Another one-liner fix, the rest is adding a test.

This has been broken since the very beginning: #2181, but probably only became relevant with Kafka sinks being introduced.

timestamp-micros: logical type annotates an Avro long, where the long stores the number of microseconds from the unix epoch, 1 January 1970 00:00:00.000000.

From https://avro.apache.org/docs/1.12.0/specification/#logical-types

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[DAlperin]

Good catch. Thanks!