For a query language, you can generalize to all nodes supporting all things. You will be able to write some queries that will never return results, at least in a given input data format. That does not seem so bad.

Coincidentally, this is exactly what XPath and XQuery do. E.g. text nodes don't have children nor attributes, yet it is perfectly legal to do text()/foo or text()/@foo - and the result is an empty sequence.

If you want this kind of semantics with JSON, take a look at https://www.jsoniq.org

No it makes so much more sense to introduce nil and then constantly pepper your code with ? (Null-colasecing) Operators everywhere

(Looking at you, jq, though to be fair JSON itself supports null...)