Check for the length of the buffer to determine whether the conversion failed
instead of checking whether it's NULL because this is currently never the case
because of the code in wxString::AsCharBuf() which returns "" and not NULL in
case of conversion failure.
This at least eliminates silent data loss when saving data that can't be
converted to the current locale encoding.
Closes#16348.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76798 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775