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Context

Whenever test execution fails with exception without message, e.g.

@Test
public void testMethodWithException() throws Exception {
	throw new Exception();
}

or when test fails with NullPointerException, we encounter issues with Categories tab showing 404 Not Found error (jenkinsci/allure-plugin#193) and test page in report lacking information on exception

This pull request suggests to use Throwable class name in case of Throwable message being empty (null)

I also added new test to allure-testng adapter covering this case

Checklist

• Sign Allure CLA
• Provide unit tests