Update AdminType.php

As mentioned in https://github.com/sonata-project/SonataAdminBundle/issues/1568 and https://github.com/sonata-project/SonataAdminBundle/pull/1710 there is something wrong with '$this->getSubject();' in embedded forms. You will get always the first entry. With this change it works like a charm and checks if the Data is empty. You won't have issues with new created entries.