HHH-18770 Check if the monitoring event was null

When the JFR partial flush event is disabled (for instance when JFR is
entirely disabled) beginPartialFlushEvent() will return a null event.

Author: gtoison

hibernate-jfr/src/main/java/org/hibernate/event/jfr/internal/JfrEventManager.java

@@ -454,7 +454,7 @@ public PartialFlushEvent beginPartialFlushEvent() {
 	public void completePartialFlushEvent(
 			HibernateMonitoringEvent hibernateMonitoringEvent,
 			AutoFlushEvent event) {
-		if ( event != null ) {
+		if ( event != null && hibernateMonitoringEvent != null) {
 			final PartialFlushEvent flushEvent = (PartialFlushEvent) hibernateMonitoringEvent;
 			flushEvent.end();
 			if ( flushEvent.shouldCommit() ) {