How do you express the idea of null when the language doesn't have null and the runtime environment can diverge from its static model over a period of years or

How do you express the idea of null when the language doesn't have null and the runtime environment can diverge from its static model over a period of years or decades? If your program typechecked against Netscape Navigator in 1997, does that prove that it will run without type errors in AwesomeNewBrowser 2023? IMO null/undefined/etc are reasonable fallbacks if your runtime environment can radically diverge from its static model.