[MrTortoise]

I feel like you missed the main point of what you are saying.

Whilst existing callers who obey the contract will not be broken by this callers who depend upon this code breaking for null values will no longer do so.

so it is a breaking change for them. You have just extended their behaviour.

That said if we defined the functionality of code as being valid within a scope of input and not defined anything outside of this we are safer in a sense. That said you do require feedback to show you are out of range.

But the point I want to make is that the line of argument that attempts to ascribe changes to behaviour on values that were out of scope as breaking changes is not helpful to anyone who is trying to co-exist via contracts in order to understand what work is expected as a result of change.