fix: kafka keys might be null

Author: lsafer-meemer

kaftor/src/commonMain/kotlin/Declarations.kt

@@ -99,7 +99,7 @@ sealed interface KafkaRecord {
     val timestampType: TimestampType
     val serializedKeySize: Int
     val serializedValueSize: Int
-    val key: String
+    val key: String?
     val value: ByteBuffer
     val headers: Headers
     val leaderEpoch: Int?

kaftor/src/commonMain/kotlin/internal.kt

@@ -93,7 +93,7 @@ private data class KafkaRecordImpl(
     override val timestampType: TimestampType,
     override val serializedKeySize: Int,
     override val serializedValueSize: Int,
-    override val key: String,
+    override val key: String?,
     override val value: ByteBuffer,
     override val headers: Headers,
     override val leaderEpoch: Int?,