Fix sending empty notifications (#19589) (#19590)

- Backport #19589
  - Don't send empty notifications on read notifications API.

routers/api/v1/notify/repo.go

@@ -214,7 +214,7 @@ func ReadRepoNotifications(ctx *context.APIContext) {
 		targetStatus = models.NotificationStatusRead
 	}
 
-	changed := make([]*structs.NotificationThread, len(nl))
+	changed := make([]*structs.NotificationThread, 0, len(nl))
 
 	for _, n := range nl {
 		notif, err := models.SetNotificationStatus(n.ID, ctx.User, targetStatus)