Code review comment for ~hyask/autopkgtest-cloud:skia/always_dump_additional_params

Nothing is relying on `all-proposed` in `testinfo.json` yet, as it was not present until like last week. Moreover, setting this explicitly to 0 means taking a conscious decision to do so even if it was not specified, while `null` is more likely to imply that the option just wasn't given. It's a bit like the modern `Option` and `Result` types in Rust: the type is `int`, but it can be `null` so that you know explicitly there is no value to look for and react accordingly.