fix(notification-outbox): fall back to Target for empty notification list name

src/ScadaLink.StoreAndForward/NotificationForwarder.cs

@@ -115,7 +115,10 @@ public sealed class NotificationForwarder
 
         submit = new NotificationSubmit(
             NotificationId: message.Id,
-            ListName: payload.ListName ?? message.Target,
+            // A null OR empty/blank ListName falls back to the S&F Target — matching the
+            // empty-string guard the former SMTP handler (NotificationDeliveryService)
+            // applied, so an empty list name is never forwarded to central.
+            ListName: string.IsNullOrEmpty(payload.ListName) ? message.Target : payload.ListName,
             Subject: payload.Subject ?? string.Empty,
             Body: payload.Message ?? string.Empty,
             SourceSiteId: _sourceSiteId,

tests/ScadaLink.StoreAndForward.Tests/NotificationForwarderTests.cs

@@ -67,6 +67,39 @@ public class NotificationForwarderTests : TestKit
         Assert.True(await deliverTask);
     }
 
+    [Fact]
+    public async Task Deliver_FallsBackToTarget_WhenPayloadListNameIsEmpty()
+    {
+        var centralProbe = CreateTestProbe();
+        var forwarder = new NotificationForwarder(
+            centralProbe.Ref, "site-7", ForwardTimeout);
+
+        // A buffered payload carrying an empty-string ListName: the empty value must not
+        // be forwarded — the forwarder falls back to the S&F message Target instead.
+        var payload = JsonSerializer.Serialize(new
+        {
+            ListName = "",
+            Subject = "Pump alarm",
+            Message = "Pump 3 tripped"
+        });
+        var msg = new StoreAndForwardMessage
+        {
+            Id = "msg-empty-list",
+            Category = StoreAndForwardCategory.Notification,
+            Target = "Operators",
+            PayloadJson = payload,
+            OriginInstanceName = "Plant.Pump3",
+        };
+
+        var deliverTask = forwarder.DeliverAsync(msg);
+
+        var submit = centralProbe.ExpectMsg<NotificationSubmit>();
+        Assert.Equal("Operators", submit.ListName);
+        centralProbe.Reply(new NotificationSubmitAck(submit.NotificationId, Accepted: true, Error: null));
+
+        Assert.True(await deliverTask);
+    }
+
     [Fact]
     public async Task Deliver_ThrowsTransient_WhenAckIsNotAccepted()
     {