C3: Query error: Unknown column

I've installed the test example and get an error for every relation. The issue seems to be that C3 will update an null value by default.

I suggest to set a `doNotSaveEmpty` in the `callbackFieldLoad` for every relation field:

``` php
$arrConfig = &$GLOBALS['TL_DCA'][$objDC->table]['fields'][$objDC->field];
$arrConfig['eval']['doNotSaveEmpty'] = true;
```
