admin/settings: Fix undefined variable warnings

There's clearly an issue with using $_GET all around Chamilo.
We need a pattern or guideline to prevent these kind of things :-(.

These warnings pop in my freshly installed Chamilo:
http://localhost:8000/chamilo-lms/main/admin/settings.php?category=Templates

```
Notice: Undefined index: action in /Users/diego/Sites/chamilo-lms/main/admin/settings.lib.php on line 723
Notice: Undefined index: action in /Users/diego/Sites/chamilo-lms/main/admin/settings.lib.php on line 729
Notice: Undefined index: action in /Users/diego/Sites/chamilo-lms/main/admin/settings.lib.php on line 729
Notice: Undefined index: action in /Users/diego/Sites/chamilo-lms/main/admin/settings.lib.php on line 738
```