The default property from dcg has an empty value handling. There are two methods evaluate for the empty value handling. The methods are hasEmptyValue and getEmptyValue. The attribute must set the values for the empty value handling.

Here can you where the empty value evaluate it.

https://github.com/contao-community-alliance/dc-general/blob/a6a0d53aa346d25a573e94ba510548271b890a32/src/Data/ModelManipulator.php#L85