But if you have an evolving API, you are back to what was asked. Either you are a bit of a jerk to all of your existing clients, or you have default values at the boundary.

I've found myself with this a lot on "progress" data classes where I will be building up a set of data over several expensive calls. I /could/ make a new type per current state of that, such that I never have nulls. I /could/ make it so that every field is Optional. Or, I could work on a convention that fields are filled out in order, and after the first null, nothing is available. In many years, this has not been where null pointers hit me.