Read empty values with boost::spirit

I want to read a CSV into a struct :

struct data 
{
   std::string a;
   std::string b;
   std::string c;
}

However, I want to read even empty string to ensure all values are in their proper place. I adapted the struct to a boost::fusion, so the following works :

// Our parser (using a custom skipper to skip comments and empty lines )
template <typename Iterator, typename skipper = comment_skipper<Iterator> >
  struct google_parser : qi::grammar<Iterator, addressbook(), skipper>
{
  google_parser() : google_parser::base_type(contacts, "contacts")
  {
    using qi::eol;
    using qi::eps;
    using qi::_1;
    using qi::_val;
    using qi::repeat;
    using standard_wide::char_;
    using phoenix::at_c;
    using phoenix::val;

    value = *(char_ - ',' - eol) [_val += _1];

    // This works but only for small structs
    entry %= value >> ',' >> value >> ',' >> value >> eol;
  }

  qi::rule<Iterator, std::string()> value;
  qi::rule<Iterator, data()> entry;
};

Unfortunately, repeat stores in a vector all non-empty values so the values of attributes may be mixed together (i.e if the field for b is null, it may contains the content from c):

    entry %= repeat(2)[ value >> ','] >> value >> eol;

I would like to use a short rule similar to repeat as my struct has 60 attributes in practice ! Not only is writing 60 rules tedious but it seems Boost does not like long rules...