Using optional directive in string-like parser results in null ('\0') character being encoded in the result

[saki7]

This behavior is currently produced by design, but it is very hard to debug. I think we should reconsider and make the string parser silently omit the element when the underlying parser produces an optional attribute.

Consider a markdown-like paragraph:

constexpr auto section_text_def = as<std::string>(
    +(
        lexeme[!lit('#') >> +(char_ - eol)] >>

        // always output the first eol as LF
        ((eol | eoi) >> attr('\n')) >>

        // squash subsequent eols to single LF
        // -(+eol >> attr('\n'))   // <--- this outputs '\0'
        (+eol >> attr('\n') | eps) // <--- workaround
    )
);

[saki7]

I think this is a bug, needs investigation.