Fix MessageFailedPublisher failing on deleted FailedMessage

In some scenarios the ExpiredDocumentCleaner process could remove a
FailedMessage prior to it being dispatched.

Author: Greg Bielleman

src/ServiceControl/ExternalIntegrations/MessageFailedPublisher.cs

@@ -19,9 +19,10 @@ protected override DispatchContext CreateDispatchRequest(MessageFailed @event)
 
         protected override IEnumerable<object> PublishEvents(IEnumerable<DispatchContext> contexts, IDocumentSession session)
         {
+            
             var documentIds = contexts.Select(x => x.FailedMessageId).Cast<ValueType>().ToArray();
-            var failedMessageData = session.Load<FailedMessage>(documentIds);
-            return failedMessageData.Select(x => x.ToEvent());
+            var failedMessageData = session.Load<FailedMessage>(documentIds); 
+            return failedMessageData.Where(p => p != null).Select(x => x.ToEvent());
         }
 
         public class DispatchContext