Null for exception message results in NPE on RepublishMessageRecoverer

[furlaneto]

In RepublishMessageRecoverer#truncateIfNecessary when the message exception is null results in NPE.

java.lang.NullPointerException: null
	at org.springframework.amqp.rabbit.retry.RepublishMessageRecoverer.truncateIfNecessary(RepublishMessageRecoverer.java:213) ~[spring-rabbit-2.2.3.RELEASE.jar:2.2.3.RELEASE]
	at org.springframework.amqp.rabbit.retry.RepublishMessageRecoverer.processStackTrace(RepublishMessageRecoverer.java:206) ~[spring-rabbit-2.2.3.RELEASE.jar:2.2.3.RELEASE]
	at org.springframework.amqp.rabbit.retry.RepublishMessageRecoverer.recover(RepublishMessageRecoverer.java:158) ~[spring-rabbit-2.2.3.RELEASE.jar:2.2.3.RELEASE]

Example of code:

RepublishMessageRecoverer recoverer = new RepublishMessageRecoverer(amqpTemplate);
Throwable nullMessageCause = new Exception(new Exception());
recoverer.recover(message, nullMessageCause);

Versions: 2.2.3 +