Results differ based on type when parsing a std::optional<T> with default_value std::nullopt

When writing to a `std::optional<T>`, CLI11 has different behavior when writing a `std::nullopt` as the `.default_val()` depending on the type. In this example, having `std::nullopt` as the default for a `std::optional<int>` works as expected (after parsing and using the default, the variable has no value). However, when writing to a `std::optional<std::string>`, the empty string `""` is written to the variable, resulting in the optional having an assigned value. ```c++ #include "CLI11.hpp" #include <optional> int main(int argc, char** argv) { CLI::App app{"Example"}; std::optional<int> a; app.add_option("-a", a) ->default_val(std::nullopt); std::optional<std::string> b; app.add_option("-b", b) ->default_val(std::nullopt); CLI11_PARSE(app, argc, argv); std::cout << "a: " << (a.has_value() ? std::to_string(a.value()) : "std::nullopt") << std::endl; std::cout << "b: " << (b.has_value() ? b.value() : "std::nullopt") << std::endl; if(b.has_value()){ std::cout << b.value().size() << " " << b.value() << " " << (b == "") << std::endl; } } ``` Compile: ```sh g++ -std=c++17 optional.cpp -o optional ``` Run ``` # Note: nothing passed in, so the defaults are used ./optional ``` Output: ``` a: std::nullopt b: 0 1 ``` Ideally, there would be a uniform behavior when writing a `std::nullopt` as a default value.