alt Hacker NewsIt doesn't have to be a breaking change -- an empty result set could still be allowed. It could simply be a perf improvement that avoids calling an expensive function with an empty result set, when it is known that the function is a no-op in this case.
If it's not a breaking change, why would a unit test fail as a result, whether or not using mocks/fakes for the code not under test? Unit tests should test the contract of a unit of code. Testing implementation details is better handled with assertions, right?
If the code being mocked changes its invariants the code under test that depends on that needs to be carefully re-examined. A failing unit test will alert one to that situation.
(I'm not being snarky, I don't understand your point and I want to.)