update selection test with null selection being invalid

IPython/html/tests/casperjs/test_cases/widgets_selection.js

@@ -76,7 +76,7 b' casper.notebook_test(function () {'
             'Widget list exists.');
 
         // Verify that no items are selected.
-        this.test.assert(verify_selection(this, -1), 'No items selected.');
+        this.test.assert(verify_selection(this, 0), 'Default first item selected.');
     });
 
     index = this.append_cell(