“Zero” is a very different concept from “none.”

Human civilization had a word for “nothing” / “none” / “not any” for as long as we’ve had language. “None” is something chimps can understand easily. Etc.

But zero—that is a counting number. Numbers themselves are a very abstract technique that required specific situations (shepherds counting sheep, etc.) to invent. And zero came much later still.

Which is to say: sum types are easy. Types with null values are hard ;)

I would bet that these bees aren’t so much reading this data as “zero”, but rather “presence/absence”, where absence is okay and presence is bad. (It’s not like they first had to do a task that trained them to find the card with a three, after all.)