Fix js issue when displaying stacktraces in report

The issue was causing the following JS error :
Uncaught TypeError: Cannot read property 'length' of undefined

Author: flolom

html-report/src/components/TestItem.js

@@ -66,7 +66,7 @@ export default class TestItem extends Component {
             </ul>
           </div>}
 
-          { !!data.stacktrace.length && <div className="card">
+          { !!data.stacktrace && <div className="card">
             <div className="title-common">Stacktrace</div>
             <pre className="row" style={ { overflow: 'auto' } }>{ data.stacktrace }</pre>
           </div>}