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

chrome/browser/notifications/notification_ui_manager_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/notifications/notification_ui_manager_mac.h"
 
 #include "base/mac/cocoa_protocols.h"
 #include "base/mac/mac_util.h"
 #include "base/sys_string_conversions.h"
 #include "chrome/browser/notifications/notification.h"
@@ skipping 110 matching lines @@
 
 NotificationUIManagerMac::~NotificationUIManagerMac() {
   CancelAll();
 }
 
 void NotificationUIManagerMac::Add(const Notification& notification,
                                    Profile* profile) {
   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 =
         [[NSClassFromString(@"NSUserNotification") alloc] init];
 
     ns_notification.title = base::SysUTF16ToNSString(notification.title());
     ns_notification.subtitle =
         base::SysUTF16ToNSString(notification.display_source());
     ns_notification.informativeText =
         base::SysUTF16ToNSString(notification.body());
@@ skipping 153 matching lines @@
     notification->Click();
 }
 
 - (BOOL)userNotificationCenter:(NSUserNotificationCenter*)center
      shouldPresentNotification:(id<CrUserNotification>)nsNotification {
   // Always display notifications, regardless of whether the app is foreground.
   return YES;
 }
 
 @end