You can't fix this in the activator because the module doesn't even load in 1.5. The sqldiff is run before the activator and the sqldiff is where the problem is. The uuid column is a "non-null unique" column and must be filled in when creating those rows.
Doing sql updates in the activator is not recommended anyway. The solution is to have a 1.4.x-compatible branch off of your current code in your module. Then in trunk its 1.5+ compatible, so you set the openmrs version in your config.xml to 1.5.0.
Marking TRAC-1994 as a duplicate of this ticket.
Jaime, can you attach a .patch file to this ticket for Daniel to review/apply?