Alerting: Notification channel http api fixes (#16379)

Fixes so it's possible to create new notification channel and providing uid.
Fixes better error/result handling when updating a notifcation channel.

Fixes #16372
Ref #16219 #16012

pkg/api/alerting.go

@@ -261,6 +261,10 @@ func UpdateAlertNotification(c *m.ReqContext, cmd m.UpdateAlertNotificationComma
 		return Error(500, "Failed to update alert notification", err)
 	}
 
+	if cmd.Result == nil {
+		return Error(404, "Alert notification not found", nil)
+	}
+
 	return JSON(200, dtos.NewAlertNotification(cmd.Result))
 }
 
@@ -272,6 +276,10 @@ func UpdateAlertNotificationByUID(c *m.ReqContext, cmd m.UpdateAlertNotification
 		return Error(500, "Failed to update alert notification", err)
 	}
 
+	if cmd.Result == nil {
+		return Error(404, "Alert notification not found", nil)
+	}
+
 	return JSON(200, dtos.NewAlertNotification(cmd.Result))
 }