Bug 1390428 (part 8) - Remove nsXPIDLCString use in NullCString(). r=erahm.

This change makes NullCString() work the same way as NullString().

--HG--
extra : rebase_source : 3835cd75f1cf95c9d0884cb9a73250123129dd07

xpcom/string/nsReadableUtils.cpp

@@ -1286,7 +1286,7 @@ NullString()
 const nsCString&
 NullCString()
 {
-  static const nsXPIDLCString sNull;
+  static const nsCString sNull(mozilla::detail::StringDataFlags::VOIDED);
 
   return sNull;
 }