EjbTimerXmlPersister should check if timer.getNextExpiration() is null to avoid NPE

Seeing a NPE in org.jboss.as.ejb3.timerservice.persistence.filestore.EjbTimerXmlPersister. that probably needs a null check:

Caused by: java.lang.NullPointerException
at org.jboss.as.ejb3.timerservice.persistence.filestore.EjbTimerXmlPersister.writeCalendarTimer(EjbTimerXmlPersister.java:136)
at org.jboss.as.ejb3.timerservice.persistence.filestore.EjbTimerXmlPersister.writeContent(EjbTimerXmlPersister.java:89)
at org.jboss.as.ejb3.timerservice.persistence.filestore.EjbTimerXmlPersister.writeContent(EjbTimerXmlPersister.java:43)
at org.jboss.staxmapper.XMLMapperImpl.doDeparse(XMLMapperImpl.java:88)
at org.jboss.staxmapper.XMLMapperImpl.deparseDocument(XMLMapperImpl.java:83)
at org.jboss.as.ejb3.timerservice.persistence.filestore.FileTimerPersistence.writeFile(FileTimerPersistence.java:440)