Sometimes the null-object pattern is a good pattern: for example, in views, when you need to show default text when an object is missing.
But the rest of the time, it is a bad pattern, or an anti-pattern.
I think that pushing the null-object pattern into ActiveRecord is a mistake. It is not an active-record concern; it is not a domain-model concern; it is a view concern only, a “last-mile” concern if you will.
This pattern sometimes appears to solve a problem of code which could be made more concise, if only ________.
But when actually applied consistently and automatically throughout a codebase at the domain-model layer, it will create more problems than it solves, and the problems it creates will be worse than the problems that it solves.
The following visceral analogy seems to me to apply: it is like solving the problem of a stubbed toe by hacking off your whole foot.
You can of course use null-objects explicitly in your views, without pushing them into ActiveRecord. And there are many other solutions too.