Fix NPE running Gradle tests with forkEvery=1

[wrprice]

With the forkEvery=N option, not all Gradle test events go through the caseStart
path, meaning that a given session may not have a testSuite map within runningSuites
hence the NPE. The extra null check here (and probably also prior work from #8879) restores
the Test Results window functionality.

Was able to reproduce the original issue and verify this fixes #6000

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[mbien]

makes sense to me, thanks for the fix!