During transition of the Windows runner image from version 20240122.1.0 to 20240128.1.0, the "Run functional tests" step in the "Win64 native" job stopped working for pull requests:

WARNING! Test '' not found in full test list.
No valid test scripts specified. Check that your test is in one of the test lists in test_runner.py, or run test_runner.py with no arguments to run all tests

It seems, that the empty value of the TEST_RUNNER_EXTRA environment variable is not handled properly.