Why do we have unit tests for things that can never happen

So you finally have established good development practices for your team. Linters enforce code coverage of 90% and everyone is adding tests. Month over month test count increases and obviously code quality is increasing as well…

Until a partner contributing code to your project notices a test that validates something that can never happen. This test in particular validates the behavior of an endpoint when a particular input is null. The issue is that our platform ensures this input is never null, any requests to our service without that input will be a platform error, and never make a request to our API. 

Our slack chat room had a lengthy discussion on why we have that unit test, but the answer is code coverage. Someone added it because code coverage declared that code path to be untested. 

Can we delete this test? Probably not without writing new tests for other functionality or reducing our coverage requirement. 

One option here is to add a Spring MethodValidationInterceptor which validates ‘base’ inputs to our service and then write tests for that. This approach moves responsibility for ‘things that can never happen’ to a centralized class which can be tested. Then we know what is prevented from happening within the domain of our unit tests. 

But at the platform level this really will never happen. Do we really need every service in our dependency tree to validate this particular case and throw an exception?