Fixes undefined array causing warnings to log.

Got a warning on `/admin/review`:

```
Notice: Undefined variable: favorite_talks in /var/www/html/opencfp/classes/OpenCFP/Entity/Mapper/Talk.php on line 73

Warning: Invalid argument supplied for foreach() in /var/www/html/opencfp/classes/OpenCFP/Entity/Mapper/Talk.php on line 73
```

Most epic fix ever... :cake: