fix(sms): repair S1 build breaks — null-filter EmailAddress projections + SiteNotificationRepository SMS stubs

S1 made NotificationRecipient.EmailAddress nullable + added SmsConfiguration
and four INotificationRepository SMS methods, breaking compilation beyond the
intentionally-deferred central NotificationRepository.

Fix 1 (CS8620/CS8604 nullable EmailAddress projections, email-only paths):
- NotificationOutbox EmailNotificationDeliveryAdapter: filter non-null emails
- DeploymentManager ArtifactDeploymentService: filter non-null emails
- Transport EntitySerializer: filter non-null emails into NotificationRecipientDto

Fix 2 (CS0535): stub the four SMS-config methods on SiteRuntime
SiteNotificationRepository (central-only — NotSupportedException, matching the
existing 'Managed via artifact deployment from Central' write-path pattern).

Doc nits: reword NotificationRecipient private ctor and SmsConfiguration.AuthToken
comments.

The central ConfigurationDatabase.NotificationRepository compile break is left
as-is (S2 implements those four methods).

src/ZB.MOM.WW.ScadaBridge.Transport/Serialization/EntitySerializer.cs

@@ -138,9 +138,11 @@ public sealed class EntitySerializer
             NotificationLists: aggregate.NotificationLists.Select(nl => new NotificationListDto(
                 Name: nl.Name,
                 Type: nl.Type,
-                Recipients: nl.Recipients.Select(r => new NotificationRecipientDto(
-                    Name: r.Name,
-                    EmailAddress: r.EmailAddress)).ToList())).ToList(),
+                Recipients: nl.Recipients
+                    .Where(r => r.EmailAddress is not null)
+                    .Select(r => new NotificationRecipientDto(
+                        Name: r.Name,
+                        EmailAddress: r.EmailAddress!)).ToList())).ToList(),
             SmtpConfigs: aggregate.SmtpConfigurations.Select(smtp =>
             {
                 SecretsBlock? secrets = null;