Avoid NPE if confirmed_flush_lsn is null

ERROR || WorkerSourceTask{id=connect_staging_job_transaction-0} Task threw an uncaught and unrecoverable exception [org.apache.kafka.connect.runtime.WorkerTask]
java.lang.NullPointerException
at org.postgresql.replication.LogSequenceNumber.valueOf(LogSequenceNumber.java:43)
at io.debezium.connector.postgresql.connection.PostgresConnection.lambda$readReplicationSlotInfo$3(PostgresConnection.java:118)
at io.debezium.jdbc.JdbcConnection.prepareQuery(JdbcConnection.java:427)
at io.debezium.connector.postgresql.connection.PostgresConnection.readReplicationSlotInfo(PostgresConnection.java:107)
at io.debezium.connector.postgresql.connection.PostgresReplicationConnection.initReplicationSlot(PostgresReplicationConnection.java:83)
at io.debezium.connector.postgresql.connection.PostgresReplicationConnection.<init>(PostgresReplicationConnection.java:73)
at io.debezium.connector.postgresql.connection.PostgresReplicationConnection.<init>(PostgresReplicationConnection.java:36)
at io.debezium.connector.postgresql.connection.PostgresReplicationConnection$ReplicationConnectionBuilder.build(PostgresReplicationConnection.java:339)
at io.debezium.connector.postgresql.PostgresTaskContext.createReplicationConnection(PostgresTaskContext.java:80)
at io.debezium.connector.postgresql.RecordsStreamProducer.<init>(RecordsStreamProducer.java:75)
at io.debezium.connector.postgresql.PostgresConnectorTask.start(PostgresConnectorTask.java:117)
at org.apache.kafka.connect.runtime.WorkerSourceTask.execute(WorkerSourceTask.java:157)
at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:170)
at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:214)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)