Defending against corrupt builds

[Vlatombe]

I've seen some builds failing during splitTests execution with an exception like

java.lang.NullPointerException: Cannot invoke "hudson.model.Run.getRootDir()" because "this.run" is null
	at hudson.tasks.junit.TestResultAction.getDataFile(TestResultAction.java:139)
	at hudson.tasks.junit.TestResultAction.load(TestResultAction.java:233)
	at hudson.tasks.junit.TestResultAction.getResult(TestResultAction.java:150)
	at hudson.tasks.junit.TestResultAction.getResult(TestResultAction.java:66)
	at org.jenkinsci.plugins.parallel_test_executor.ParallelTestExecutor.getTestResult(ParallelTestExecutor.java:372)
	at org.jenkinsci.plugins.parallel_test_executor.ParallelTestExecutor.findPreviousTestResult(ParallelTestExecutor.java:349)
	at org.jenkinsci.plugins.parallel_test_executor.ParallelTestExecutor.findTestSplits(ParallelTestExecutor.java:196)
	at org.jenkinsci.plugins.parallel_test_executor.SplitStep$Execution.run(SplitStep.java:139)
	at org.jenkinsci.plugins.parallel_test_executor.SplitStep$Execution.run(SplitStep.java:116)
	at org.jenkinsci.plugins.workflow.steps.SynchronousStepExecution.start(SynchronousStepExecution.java:37)
[...]

There may be a bug in the JUnit plugin, or some problem with pipeline persistence that causes it to load the build partially corrupted. In any case, the plugin should defend itself against such state and skip such build.

I don't know how to recreate such a broken run state, so no added test for now.

Testing done

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[Vlatombe]

(ignore ws)

[jglick]

Following up #247.