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

base/sys_string_conversions.h

 // 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.
 
 #ifndef BASE_SYS_STRING_CONVERSIONS_H_
 #define BASE_SYS_STRING_CONVERSIONS_H_
 
 // Provides system-dependent string type conversions for cases where it's
 // necessary to not use ICU. Generally, you should not need this in Chrome,
 // but it is used in some shared code. Dependencies should be minimal.
@@ skipping 53 matching lines @@
 // Same, but returns an autoreleased NSString.
 NSString* SysUTF8ToNSString(const std::string& utf8);
 NSString* SysUTF16ToNSString(const string16& utf16);
 NSString* SysWideToNSString(const std::wstring& wide);
 
 // Converts a CFStringRef to an STL string. Returns an empty string on failure.
 std::string SysCFStringRefToUTF8(CFStringRef ref);
 string16 SysCFStringRefToUTF16(CFStringRef ref);
 std::wstring SysCFStringRefToWide(CFStringRef ref);
 
-// Same, but accepts NSString input.
+// Same, but accepts NSString input. Converts nil NSString* to the appropriate
+// string type of length 0.
 std::string SysNSStringToUTF8(NSString* ref);
 string16 SysNSStringToUTF16(NSString* ref);
 std::wstring SysNSStringToWide(NSString* ref);
 
 #endif  // defined(OS_MACOSX)
 
 }  // namespace base
 
 #endif  // BASE_SYS_STRING_CONVERSIONS_H_