KIP-1140 — Avoid to return null value in Map from public api of consumer

Discussion Consumer

Changes `KafkaConsumer.offsetsForTimes()` to return `Optional<OffsetAndTimestamp>` instead of `OffsetAndTimestamp` (nullable) for partitions with no matching timestamp, removing null values from the returned map. Returning null values in a `Map` violates the principle of least surprise and causes `NullPointerException` when callers iterate the map without null checks.

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