test(notification-outbox): assert NotNull in SourceScript negative test

tests/ScadaLink.SiteRuntime.Tests/Scripts/NotifyHelperTests.cs

@@ -142,6 +142,7 @@ public class NotifyHelperTests : TestKit, IAsyncLifetime, IDisposable
         var notificationId = await notify.To("Operators").Send("Pump alarm", "Pump 3 tripped");
 
         var buffered = await _saf.GetMessageByIdAsync(notificationId);
+        Assert.NotNull(buffered);
         var payload = JsonSerializer.Deserialize<NotificationSubmit>(buffered!.PayloadJson);
         Assert.Null(payload!.SourceScript);
     }