There is no one solution to null. That's the point: "null" is the billion-dollar mistake because it solves so many different problems in exactly the same way.

To fix it, you have to go back and reconsider what it means for your data to lack that value. Is it a different type of thing? Does it mean that the value must exist but isn't known? Does it mean that it's really some default value? Does it mean that there's an error but you're just trying to muddle along? Does it mean you could actually have zero or more of something? Does it mean it's uninitialized?

You might try to solve any of these with a null. That's simple, but it does bad things to your type system. At least using sentinels makes it easier to reason about your type system, but if you're just using it as a "typed null" then you've just hidden the problem further.

I find that the better you design your types to model the real problem domain, the less you need nulls or sentinels or other hacks. It all comes together naturally. Not always, but any time it doesn't, it's a smell.