[SPARK-7534] [CORE] [WEBUI] Fix the Stage table when a stage is missing

Just improved the Stage table when a stage is missing.

Before:

![screen shot 2015-05-11 at 10 11 51 am](https://cloud.githubusercontent.com/assets/1000778/7570842/2ba37380-f7c8-11e4-9b5f-cf1a6264b2a4.png)

After:

![screen shot 2015-05-11 at 10 26 09 am](https://cloud.githubusercontent.com/assets/1000778/7570848/33703152-f7c8-11e4-81a8-d53dd72d7b8d.png)

Author: zsxwing <zsxwing@gmail.com>

Closes #6061 from zsxwing/SPARK-7534 and squashes the following commits:

09fe862 [zsxwing] Leave it blank rather than '-'
6299197 [zsxwing] Fix the Stage table when a stage is missing