Fix handling null messages during timeout evaluation.

[OlgaMaciaszek]

We are getting NPEs for certain scenarios over at Spring Cloud Netflix due to lack of handling of null exception messages. That's what gets logged:

java.lang.NullPointerException: Cannot invoke "String.toLowerCase()" because the return value of "java.lang.Throwable.getMessage()" is null
	at com.netflix.eureka.cluster.ReplicationTaskProcessor.maybeReadTimeOut(ReplicationTaskProcessor.java:196) ~[eureka-core-2.0.0.jar:2.0.0]
	at com.netflix.eureka.cluster.ReplicationTaskProcessor.process(ReplicationTaskProcessor.java:95) ~[eureka-core-2.0.0.jar:2.0.0]
	at com.netflix.eureka.util.batcher.TaskExecutors$BatchWorkerRunnable.run(TaskExecutors.java:190) ~[eureka-core-2.0.0.jar:2.0.0]
	at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]

Since I've seen AssertJ was already being used in the project, I have added it to the module where I've made modifications. While at it, have also upgraded the AssertJ version.

If you accept this PR, could you also merge this change into 2.x?