I like your idea @mbillion of a “zero quantity harvest”… it could work for you anyway. Until we have a more hard-defined way to represent this in the data model, this is largely a “convention” question.
In other words: as long as you stick to the convention you develop, you’ll have consistent data. We’ve been talking a lot of about the idea of “conventions” more generally… and how they can “bubble up” to create more official designations in the data model itself as folks come to agreement (like in this topic!).
See: Creating a standard 'interpretation' layer on top of core farmOS schema
And this draft I’m working on in the data model documentation: farmOS/index.md at 2.x-docs · mstenta/farmOS · GitHub