Unable to read messages from RabbitMQ as a continuous stream

The RabbitMqIO connector does not work entirely as expected. Not passing `withMaxReadTime` gives a NullPointer exception. Exact error: 
```

Exception in thread "main" java.lang.NullPointerException
at org.apache.beam.runners.direct.UnboundedReadEvaluatorFactory$UnboundedReadEvaluator.processElement(UnboundedReadEvaluatorFactory.java:169)
at
org.apache.beam.runners.direct.DirectTransformExecutor.processElements(DirectTransformExecutor.java:160)
at
org.apache.beam.runners.direct.DirectTransformExecutor.run(DirectTransformExecutor.java:124)
at java.util.concurrent.Executors$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)
 
```

It seems RabbitMqIO does not cover the case when messages received from RabbitMQ have messages of which timestamps are null. On Stackoverflow the following workaround is proposed:
[https://stackoverflow.com/a/54119782](https://stackoverflow.com/a/54119782)

Imported from Jira [BEAM-6515](https://issues.apache.org/jira/browse/BEAM-6515). Original Jira may contain additional context.
Reported by: edin.