The xunit file is malformed int the passing case (0 tests)

[asymingt]

We noticed in CI that when ament_black tests pass our build goes green, but the interface that scrapes the XML file doesn't list any of the passing tests from black. This is a direct consequence of the fact that report argument passed to the xunit construction logic is empty, because it is built from the PatchSet, which in the passing case is empty.

Example from a test passing with 26 files:

<?xml version="1.0" encoding="UTF-8"?>
<testsuite  name="my_test_case.black" 
                   tests="0" 
                   errors="0" 
                   failures="0" 
                   time="0.233" >
    <system-out>Checked files:</system-out>
</testsuite>

This should be fixed to be brought inline with the other ROS linting tools.