#1 null input for non generated IDs crash

[Remi Vankeisbelck]

If a POJO has an ID field that's not automatically generated, and the user
does not input any value for it in the edit mode, then a nasty hibernate
exception is thrown when trying to save the object.

The bug is easily repeatable :
* log in as wdevel
* get to the users/roles page
* select a role, and then a user
* edit it
* clear its "username" prop (it's the ID) and save

booooom !