If your SQL model has nulls, and you don't have some clear way to conserve them throughout the data chain, including to the json schema in your API contract, you have a bug. That way to preserve them doesn't have to be keeping nulls distinct from missing values in the json schema, but it's certainly the most straightforward way.

The world has more than three languages, and the way Java and Python do things is not universally correct. I'm not up to date on either of them, but I'm also guessing that they both have multiple libraries for (de) serialization and for API contract validation, so I am not really convinced your claims are universal even within those languages.

I am not the other person you were talking to, I've only made one comment on this, so not really "hellbent", friend.

Yes, I am pretty sure I read the comments, although you're making me wonder if I'm missing one. What specific comment, what "case specified above" are you referring to? As far as I can see, you are the one trying to say that if a distinction between null and a non-existent attribute is not specified, it should universally be assumed to be meaningless and fine to drop null values. I don't see any context that changes that. If you can point it out, specifically, I'll be glad to reassess.