Ensure that conversions from/to 8bit and 16bit never return null string.

Source/wtf/text/WTFString.cpp

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007-2009 Torch Mobile, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 856 matching lines @@
             }
         }
     }
 
     return CString(bufferVector.data(), buffer - bufferVector.data());
 }
 
 String String::make8BitFrom16BitSource(const UChar* source, size_t length)
 {
     if (!length)
-        return String();
+        return emptyString();
 
     LChar* destination;
     String result = String::createUninitialized(length, destination);
 
     copyLCharsFromUCharSource(destination, source, length);
 
     return result;
 }
 
 String String::make16BitFrom8BitSource(const LChar* source, size_t length)
 {
     if (!length)
-        return String();
+        return emptyString();
 
     UChar* destination;
     String result = String::createUninitialized(length, destination);
 
     StringImpl::copyChars(destination, source, length);
 
     return result;
 }
 
 String String::fromUTF8(const LChar* stringStart, size_t length)
@@ skipping 365 matching lines @@
     buffer.append('\0');
     return buffer;
 }
 
 Vector<char> asciiDebug(String& string)
 {
     return asciiDebug(string.impl());
 }
 
 #endif