### What changes were proposed in this pull request?
This PR proposes to skip test reporting ("Report test results") if there are no JUnit XML files are found.
Currently, we're running and skipping the tests dynamically. For example,
- if there are only changes in SparkR at the underlying commit, it only runs the SparkR tests, and skip the other tests and generate JUnit XML files for SparkR test cases.
- if there are only changes in `docs` at the underlying commit, the build skips all tests except linters and do not generate any JUnit XML files.
When test reporting ("Report test results") job is triggered after the main build ("Build and test
") is finished, and there are no JUnit XML files found, it reports the case as a failure. See https://github.com/apache/spark/runs/1196184007 as an example.
This PR works around it by simply skipping the testing report when there are no JUnit XML files are found.
Please see https://github.com/apache/spark/pull/29906#issuecomment-702525542 for more details.
### Why are the changes needed?
To avoid false alarm for test results.
### Does this PR introduce _any_ user-facing change?
No, dev-only.
### How was this patch tested?
Manually tested in my fork.
Positive case:
https://github.com/HyukjinKwon/spark/runs/1208624679?check_suite_focus=truehttps://github.com/HyukjinKwon/spark/actions/runs/288996327
Negative case:
https://github.com/HyukjinKwon/spark/runs/1208229838?check_suite_focus=truehttps://github.com/HyukjinKwon/spark/actions/runs/289000058Closes#29946 from HyukjinKwon/test-junit-files.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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