At the (SQL) database level, if you are using null in any sane way, it means “this value exists but is unknown”. Conflating that with “this value does not exist” is very dangerous. JavaScript, the closest thing there is to a reference implementation for json serialization, drops attributes set to undefined, but preserves null. You seem to be insisting that null only means “explicit omission”, but that isn’t the case. Null means a variety of subtly different things in different contexts. It’s perfectly fine to explicitly define null and missing as equivalent in any given protocol, but assuming it is not.
You seem to have missed the important phrase “in source code”, as well as the entire second part of my comment discussing that runtime functions that parse user input are different.