Nothing obvious that would speak against converting from the original
SvStringsDtor to plain std::vector<...> instead of slavishly sticking to
boost::ptr_vector<boost::nullable<...>>.  Also, prevents a false -fsanitize=null
warning about "reference binding to null pointer of type 'rtl::OUString'" in
boost::void_ptr_iterator<...>::operator*() during ~SvxClipboardFmtItem_Impl().

Change-Id: I08d8fc573ff99a5bddd67c0d2be4e7ea38025958