Coverage job produces empty reports
Since the recent changes to the coverage job (!291 (merged), !296 (merged)), the nightly run produces empty coverage reports.
https://i10git.cs.fau.de/walberla/walberla/-/jobs/406087:
$ gcovr -r $CI_PROJECT_DIR -f ".*\\/src\\/.*" -k
2711 ------------------------------------------------------------------------------
2712 GCC Code Coverage Report
2713 Directory: /builds/walberla/walberla
2714 ------------------------------------------------------------------------------
2715 File Lines Exec Cover Missing
2716 ------------------------------------------------------------------------------
2717 ------------------------------------------------------------------------------
2718 TOTAL 0 0 --%
2719 ------------------------------------------------------------------------------