Requesting rejection by null subscriber is logged when the subscriber is not acknowledging

[pamod]

Steps

1. Create a queue - Test (0 messages)
2. Create subscriber to send acknowledgements and subscribe to Test queue
3. Publish messages to the Test queue
4. Make sure the subscriber won't send acknowledgements. Issue

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"Requesting rejection by null subscriber" is printed in the logs where null does not describe which subscriber rejects the messages Expected

Provide meaningful details of the subscriber without logging NULL

Referred from https://wso2.org/jira/browse/MB-1264