When a user requests a code review, the review is responsible for making sure that the code is tested. While the quality of the tests is a subjective matter, their presences is not; either they are there or they are not there. If they are not there, it is on the developer to explain why or why not.
Not every line of code is testable. Not every test is intelligent. But, at a minimum, a test should ensure that the code in a patch is run at least once, without an unexpected exception.