fix linking problems due to MSVC implicitly considering template classes specializati...

fix linking problems due to MSVC implicitly considering template classes specializations used as base classes DLL-exported even though it doesn't actually export them when building the DLL (with or without explicit declspec(dllexport)); also replace wxCharTypeBuffer::NullData with a static function to allow not DLL-exporting this class

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56602 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775