Fix persist workflow when has no subscription

Author: glucaci

src/WorkflowCore/Services/BackgroundTasks/WorkflowConsumer.cs

@@ -55,7 +55,7 @@ protected override async Task ProcessItem(string itemId, CancellationToken cance
                     finally
                     {
                         WorkflowActivity.Enrich(result);
-                        await _persistenceStore.PersistWorkflow(workflow, result.Subscriptions, cancellationToken);
+                        await _persistenceStore.PersistWorkflow(workflow, result?.Subscriptions, cancellationToken);
                         await QueueProvider.QueueWork(itemId, QueueType.Index);
                         _greylist.Remove($"wf:{itemId}");
                     }

src/providers/WorkflowCore.Persistence.MongoDB/Services/MongoPersistenceProvider.cs

@@ -154,12 +154,18 @@ public async Task PersistWorkflow(WorkflowInstance workflow, CancellationToken c
 
         public async Task PersistWorkflow(WorkflowInstance workflow, List<EventSubscription> subscriptions, CancellationToken cancellationToken = default)
         {
-            using (var session = await _database.Client.StartSessionAsync())
+            if (subscriptions == null || subscriptions.Count < 1)
+            {
+                await PersistWorkflow(workflow, cancellationToken);
+                return;
+            }
+
+            using (var session = await _database.Client.StartSessionAsync(cancellationToken: cancellationToken))
             {
                 session.StartTransaction();
                 await PersistWorkflow(workflow, cancellationToken);
                 await EventSubscriptions.InsertManyAsync(subscriptions, cancellationToken: cancellationToken);
-                await session.CommitTransactionAsync();
+                await session.CommitTransactionAsync(cancellationToken);
             }
         }