fixes error when opcache is disabled

If you load the gui panel with opcache disabled in php.ini, you get an error

```
Fatal error: Uncaught exception 'Exception' with message 'DateTime::__construct(): Failed to parse time string (@time()) at position 0 (@): Unexpected character' in /var/www/opcache/src/OpCacheGUI/OpCache/Status.php:141 Stack trace: #0 /var/www/opcache/src/OpCacheGUI/OpCache/Status.php(141): DateTime->__construct('@time()') #1 /var/www/opcache/template/status.phtml(39): OpCacheGUI\OpCache\Status->getStatsInfo() #2 /var/www/opcache/bootstrap.php(95): require('/var/www/opcach...') #3 /var/www/opcache/public/index.php(3): require('/var/www/opcach...') #4 {main} thrown in /var/www/opcache/src/OpCacheGUI/OpCache/Status.php on line 141
```

So I added a verification to the getStatsInfo method to fallback to empty stats, displaying the following:

![rsz_screenshot_from_2014-07-26_134203](https://cloud.githubusercontent.com/assets/238439/3711816/02ae7cae-14ed-11e4-8ecc-8eccc2dbeae7.png)