Mqttv5 client throws MqttException when clean session is using Server assigned clientID with default file persistence

[harshitsharma635]

When using cleanStart = true and server assigned clientID (ClientID set to null or "") in connect packet with MqttDefaultFilePersistence we get a MqttException :

Untranslated MqttException - RC: 0 (0)
	at org.eclipse.paho.mqttv5.client.persist.MqttDefaultFilePersistence.checkIsOpen(MqttDefaultFilePersistence.java:137)
	at org.eclipse.paho.mqttv5.client.persist.MqttDefaultFilePersistence.clear(MqttDefaultFilePersistence.java:297)
	at org.eclipse.paho.mqttv5.client.internal.ClientState.clearState(ClientState.java:217)
	at org.eclipse.paho.mqttv5.client.internal.ClientState.notifyReceivedAck(ClientState.java:1062)
	at org.eclipse.paho.mqttv5.client.internal.CommsReceiver.run(CommsReceiver.java:153)
	at java.base/java.lang.Thread.run(Thread.java:1595)

• Bug exists Release Version 1.2.5 ( Master Branch)
• Bug exists in MQTTv3 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)
• Bug exists in MQTTv5 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)