Mock call verified, result never checked

Tests only verify that a collaborator was called (mock call verification) and never assert the value the unit returns. A change that hard-codes or corrupts the output passes.

The only assertion is that the repository's save was called, so the method can start returning null and the test stays green.

Not test data that never reaches the branch (Unreached Branch Test) and not an expected value computed from the code under test (Self Derived Expected Value); the branch runs and the call is seen, but the result is never checked.