don't throw exception in reverseTransform when id is empty but return null

As stated in the docblock in `DataTransformerInterface`:
> By convention, reverseTransform() should return NULL if an empty string
> is passed.

I spend too much time on a problem related to this. Sometime the entity is just empty (it's gonna be created on submit for example). I know now that when a transformer throws an exception, all you get in the form is an error message "this value is invalid" and you have no clue about what is wrong and where.