Fix NSString conversions to treat a null NSString as a string of length 0, in...

base/sys_string_conversions_mac.mm

 // Copyright (c) 2006-2008 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 "base/sys_string_conversions.h"
 
 #import <Foundation/Foundation.h>
 
 #include <vector>
 
@@ skipping 166 matching lines @@
   return CFStringToSTLStringWithEncodingT<string16>(ref,
                                                     kMediumStringEncoding);
 }
 
 std::wstring SysCFStringRefToWide(CFStringRef ref) {
   return CFStringToSTLStringWithEncodingT<std::wstring>(ref,
                                                         kWideStringEncoding);
 }
 
 std::string SysNSStringToUTF8(NSString* nsstring) {
+  if (!nsstring)
+    return std::string();
   return SysCFStringRefToUTF8(reinterpret_cast<CFStringRef>(nsstring));
 }
 
 string16 SysNSStringToUTF16(NSString* nsstring) {
+  if (!nsstring)
+    return string16();
   return SysCFStringRefToUTF16(reinterpret_cast<CFStringRef>(nsstring));
 }
 
 std::wstring SysNSStringToWide(NSString* nsstring) {
+  if (!nsstring)
+    return std::wstring();
   return SysCFStringRefToWide(reinterpret_cast<CFStringRef>(nsstring));
 }
 
 }  // namespace base