Another simplified null check

Author: DavidBoike

src/NServiceBus.NHibernate/Outbox/NHibernateLocalOutboxTransaction.cs

@@ -49,11 +49,8 @@ public async Task Commit(CancellationToken cancellationToken = default)
         public void Dispose()
         {
             //If save changes callback failed, we need to dispose the transaction here.
-            if (transaction != null)
-            {
-                transaction.Dispose();
-                transaction = null;
-            }
+            transaction?.Dispose();
+            transaction = null;
             Session.Dispose();
         }