Fix running tests with empty test set.

This was causing a crash when using an overly strict filter.

Bug: angleproject:5417
Change-Id: I38bae1a155a6b1047bae0856348de67f9cda21eb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3402939
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

src/tests/test_utils/runner/TestSuite.cpp

@@ -1275,6 +1275,7 @@ TestSuite::TestSuite(int *argc, char **argv)
         }
     }
 
+    if (!testSet.empty())
     {
         std::stringstream fakeTestName;
         fakeTestName << kArtifactsFakeTestName << '-' << testSet[0].testName;