Hub UI displays does not check if hub URL is empty

The UI can take in multiple Hub urls separated by `` and each Hub will be displayed as a separate tab in the UI. If the config has a trailing `` this will result in an empty string after the UI splits the urls and it will display a "hidden" tab that will fail to load anything (see image below)

Steps to reproduce:
1. Edit the `market.base.urls` property in cdap-site to something like : `www.example.com+`
2. Open the CDAP UI and click on Hub. If you click just to the right of the default hub you will notice there is an empty tab.