In this case we're after the "Note" field which is not always present. It seems if there is text then it's returned, otherwise it's an absent field.
If we use: [ENVIRONMENT]/[MID]/orders?returnnullfields=true then it guarantees it will be returned, but with many other fields we don't want.
I realise it's not a huge issue to handle the nulls here, but I thought I'd check just in case there is a simple API parameter I've overlooked!
Thanks