Pass empty string quote (e.g. "") as value to unregistered boost::program_options

Question

Summary

I've seen this question, that asks and answers how to pass a no-op for a defined value. However, how do you pass in an empty string to an object from command-line, especially for unregistered options? And how can I avoid boost segmentation faults as a result of passing in an empty string/value?

Details

For example, I have options that look like the following. They are deemed unregistered by the program's main entry-point. They are then passed down to another library to parse.

# These are equivalent, with and without quotes
--MyLibrary.myCsv="one,two,three"
--MyLibrary.myCsv=one,two,three

However, this option can be blank, so something like:

--MyLibrary.myCsv=""

Now, since the quotes are removed by the shell, this throws a SegFault (for versions earlier than boost 1.67, at least). To be explicit, I get an exception followed by the SegFault message "Aborted (core dumped)":

terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::program_options::invalid_command_line_syntax> >'
  what():  the argument for option '--MyLibrary.myCsv' should follow immediately after the equal sign
(core dumped)

I can write it like this:

--MyLibrary.myCsv=\"\"

And it won't seg fault, but humorously, it will sets the value to "\"\"", which is clearly not an empty string... Is there a way to just give it an empty string for value?

Example Code

Here is a small example (hopefully) showcasing my problem. I called it emptyQuotes.cpp

#include <iostream>
#include <vector>
#include <string>
#include <boost/program_options.hpp>

using namespace std;
namespace po = boost::program_options;

/**
 * @brief parser for another (potentially external) library.
 * 
 * For simplicity, it looks for '=' to split key from value
 */
void libraryParser( const vector<string>& options )
{
    for ( auto it = options.begin(); it != options.end(); ++it )
    {
        size_t eqPos = it->find('=');
        size_t start = it->find_first_not_of('-');

        string key = it->substr(start, eqPos-start);
        string val = it->substr(eqPos + 1);

        cout << "    key = \"" << key << "\"; val = \"" << val << "\"\n";
        cout << "    Is val empty? ";
        if(val.empty())
            cout << "yes";
        else
            cout << "no"; 
        cout << "\n";
    }
}

int main (int argc, char** argv)
{
    cout << "Command line:\n  ";
    for ( int ii = 0; ii < argc; ++ii )
        cout << argv[ii] << " ";
    cout << "\n";

    // Commands that the basic entry point would know
    boost::program_options::options_description basic("Basic");
    basic.add_options()
        ( "help,h", "produce a help message" );


    po::parsed_options parsed = po::command_line_parser(argc, argv)
        .options(basic)
        .allow_unregistered() // <-I don't know how to handle blank values here
        .run();

    po::variables_map vm;
    po::store(parsed, vm);
    vector<string> unregOptions = po::collect_unrecognized( parsed.options, po::exclude_positional );

    libraryParser(unregOptions);

    cout << "\n";
    return 0;
}

Running the following got the SegFault:

./emptyQuotes --MyLibrary.myVal=""
Command line:
  ./emptyQuotes --MyLibrary.myVal= 
terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::program_options::invalid_command_line_syntax> >'
  what():  the argument for option '--MyLibrary.myVal' should follow immediately after the equal sign
Aborted (core dumped)

And it reported the value wasn't empty with the slashes:

./emptyQuotes --MyLibrary.myVal=\"\"
Command line:
  ./emptyQuotes --MyLibrary.myVal="" 
    key = "MyLibrary.myVal"; val = """"
    Is val empty? no