Fix nullpointer exception when reporting with TakeAllSelector

[lacinoire]

When using the TakeAllSelector, a null pointer exception appears in the output log.

Example excerpt from the log from running MainTest..testMainWithEmptyTestMethods() on master:

[INFO] 2020-06-01 11:10:10 DSpot - elapsedTime 1310
java.lang.NullPointerException
	at eu.stamp_project.dspot.common.report.output.selector.TestSelectorElementReportImpl.getReportForCollector(TestSelectorElementReportImpl.java:42)
	at eu.stamp_project.dspot.common.configuration.AmplificationSetup.postAmplification(AmplificationSetup.java:84)
	at eu.stamp_project.dspot.DSpot.run(DSpot.java:54)
	at eu.stamp_project.Main.main(Main.java:20)
	at eu.stamp_project.MainTest.testMainWithEmptyTestMethods(MainTest.java:164)
	at ...
[ERROR] 2020-06-01 11:10:10 DSpot - Something bad happened during the report fot test-criterion.

I added a simple TestClassJSON for the TakeAllSelector, and a test case that shows that the NullPointerException is no longer appearing.

[coveralls]

Pull Request Test Coverage Report for Build 2514

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 83.223%

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Totals
Change from base Build 2513:	0.006%
Covered Lines:	4911
Relevant Lines:	5901

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💛 - Coveralls

[monperrus]

Thanks a lot:!