DBZ-7208 - implemented check for NPE

[sherpa003]

trivial fix

[Naros]

@sherpa003, what exactly is this static method called from? It seems unused from Debezium's code base.

[sherpa003]

@sherpa003, what exactly is this static method called from? It seems unused from Debezium's code base.

In our embedded debezium use case, we call it from outside periodically:
AbstractInfinispanLogMinerEventProcessor.logCacheStats();

Otherwise, the problem is that the displayCacheStatistics method is used only in constructor of event processor and nowhere else. This gives some visibility into cache state (e.g. active transactions are in the metrics but recent transactions are nowhere).