That's just the JSON equivalent of "we have data, and it's null" vs "data is missing", and consistency could be enforced using a constraint or by making it non-NULL in the first place.

It's more common in string fields, which in many cases just get rendered on a web form that doesn't differentiate blank (empty string) from null state, therefore in the database we should in most cases set it up as follows:

- Value required: field should be non-NULL + at least length 1 (via check constraint)

- Value optional: either field is non-NULL, or field is nullable + at least length 1

I'm curious if you prefer to store optional strings as non-NULL and rely on the length, or as nullable and have a length constraint.