The problem with using empty() is that if someone deletes all of the default values then it would also be an empty array and therefore you would be undoing any work that they did in deleting the values. When I first posted this code ACF would return NULL if the field was never set, this could have changed, but my assumption is still that something different should be returned for a field that has never been set than a field that has an empty value. This would need to be tested again.
