Null-check entity before observation collection

Author: acogoluegnes

src/main/java/com/rabbitmq/stream/impl/SimpleMessageAccumulator.java

@@ -75,8 +75,10 @@ public void add(Message message, ConfirmationHandler confirmationHandler) {
 
   AccumulatedEntity get() {
     AccumulatedEntity entity = this.messages.poll();
-    this.observationCollector.published(
-        entity.observationContext(), entity.confirmationCallback().message());
+    if (entity != null) {
+      this.observationCollector.published(
+          entity.observationContext(), entity.confirmationCallback().message());
+    }
     return entity;
   }