Example failed test run: https://integration.wikimedia.org/ci/job/selenium-daily-beta-MediaWiki/1016/testReport/
Example error:
Test report file /srv/jenkins/workspace/workspace/selenium-daily-beta-MediaWiki/log/WDIO.xunit-2021-04-29T16-20-00-944Z.xml was length 0
Example empty file:
I was able to reproduce this locally by running npm run selenium-daily.
There's probably some configuration to make sure junit formatter doesn't create empty files: