NIFI-4290: Ensure that the InFlightMessageTracker is instantiated when getTracker() is called

This closes #2083.

Signed-off-by: Koji Kawamura <ijokarumawak@apache.org>

nifi-nar-bundles/nifi-kafka-bundle/nifi-kafka-0-10-processors/src/main/java/org/apache/nifi/processors/kafka/pubsub/PublisherLease.java

@@ -192,6 +192,10 @@ public class PublisherLease implements Closeable {
     }
 
     public InFlightMessageTracker getTracker() {
+        if (tracker == null) {
+            tracker = new InFlightMessageTracker();
+        }
+
         return tracker;
     }
 }