fix(VerticalNav): fix attributes for secondary items (#1574)

Author: glekner

packages/patternfly-3/patternfly-react/src/components/VerticalNav/VerticalNavItemHelper.js

@@ -340,8 +340,8 @@ BaseVerticalNavItemHelper.defaultProps = {
   title: '',
   isDivider: false,
   preventHref: true,
-  id: '',
-  dataID: ''
+  id: null,
+  dataID: null
 };
 
 const VerticalNavItemHelper = getContext(navContextTypes)(BaseVerticalNavItemHelper);