avoid adding null comment in json reporter

[Atari361]

org.testng.ITestResult#getThrowable sometimes can have detailMessage=null, and this results in json-report.json having "comment":null and later a rejection from API when importing results via '/api/v2/import/execution' (Xray Cloud).

TestNG test example that generates detailMessage=null:

@Test
@XrayTest(key = "EZ-76901")
public void testFailed() {
    assert false;
}

This PR adds an extra detailMessage != null check to favoid adding null comment.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
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