rfl::NoOptionals incorrect for TOML #656

When a std::optional<struct> is written to toml with rfl::NoOptionals, it is ouput as field = '', but this causes a parse error if you try to read it back: Could not cast to a table!

I believe the correct functionality would be to write it as field = {} instead.

Note: I'm using it through using ReflectionType = std::optional<T> if that makes any difference.