Fix Undefined offset Notice

When deleting an entry a PHP notice is displayed, something like:

```
 Notice: Undefined offset: 0 in /datas/vol2/w4a141590/var/www/app.inthepoche.com/htdocs/core/inc/poche/Database.class.php on line 169
```
(it's even displayed on http://app.inthepoche.com)

This simple fix checks if the offset is set (so we have at least one entry to return), otherwise it returns null.
Vim has automatically deleted some unneeded white spaces, sorry for the noise ;)