[Mac] Only attempt to replace a notification if the replacement_id is not empty.

chrome/browser/notifications/notification_ui_manager_mac.mm

Index: chrome/browser/notifications/notification_ui_manager_mac.mm
diff --git a/chrome/browser/notifications/notification_ui_manager_mac.mm b/chrome/browser/notifications/notification_ui_manager_mac.mm
index ba74927e01f26963e541abb28ceca7f8035e0a15..cd10fd6719aaaa85949cb3da8bfc6cd5c7d3e824 100644
--- a/chrome/browser/notifications/notification_ui_manager_mac.mm
+++ b/chrome/browser/notifications/notification_ui_manager_mac.mm
@@ -128,10 +128,12 @@ void NotificationUIManagerMac::Add(const Notification& notification,
   if (notification.is_html()) {
     builtin_manager_->Add(notification, profile);
   } else {
-    id<CrUserNotification> replacee = FindNotificationWithReplacementId(
-        notification.replace_id());
-    if (replacee)
-      RemoveNotification(replacee);
+    if (!notification.replace_id().empty()) {
+      id<CrUserNotification> replacee = FindNotificationWithReplacementId(
+          notification.replace_id());
+      if (replacee)
+        RemoveNotification(replacee);
+    }
 
     // Owned by ControllerNotification.
     id<CrUserNotification> ns_notification =