DAP: Skip serializing Options when None

DAP follows the same strict TypeScript interface syntax as LSP
which states:

> The document uses TypeScript interfaces in strict mode to describe
> these. This means for example that a `null` value has to be explicitly
> listed and that a mandatory property must be listed even if a falsify
> value might exist.

So we have to skip serializing any fields that end in `?` instead
of passing `null`.