avoid encryption on null values

[gtrombi]

Hi, thanks for this very useful extension. I have a question: I have a model attribute that can be null, but if is not, then it must be unique. This check is performed at database level. As far as I understand, the encryption is performed on null fields as well, therefore producing an integrity violation at database level, since the encrypted string resulting from encrypting '' (which is the value of the blank field left as null) is actually not null. Is there already a way to prevent this, or I should work on it? Thank you very much