Fix Copy Template and Rename File buttons if Hathor is uninstalled

Based on a JSE question: http://joomla.stackexchange.com/questions/2501/where-is-the-copy-template-button-in-template-manager-in-j3-3-1-after-update

### Issue
If you uninstall Hathor, the "Copy Template" and "Rename File" buttons in the template manager is no longer showing up. This is because the current check explicitely checks for `0`. The database query however returns `null` if Hathor is uninstalled.

### Solution
Instead of checking `$this->hathor->home == 0` we just check if the value is empty, which includes `0`, `null` or an empty string. Using `empty($this->hathor->home)` also allows to remove the `$this->hathor` check since it will not generate a warning if the object property doesn't exist.

#### Testing
Uninstall Hathor and check the presence of the mentioned buttons in the template manager (when you edit a template or a file).