fix(): fixing exception in ingestion of expired keys in keys source

Author: sauravkumarrr

core/redis-kafka-connect/src/main/java/com/redis/kafka/connect/source/ToStructFunction.java

@@ -39,7 +39,7 @@ public class ToStructFunction implements Function<KeyValue<String>, Struct> {
 
 	public static final Schema TTL_SCHEMA = Schema.OPTIONAL_INT64_SCHEMA;
 
-	public static final Schema TYPE_SCHEMA = Schema.STRING_SCHEMA;
+	public static final Schema TYPE_SCHEMA = Schema.OPTIONAL_STRING_SCHEMA;
 
 	public static final String VALUE_SCHEMA_NAME = "com.redis.kafka.connect.keys.Value";