Allow empty default group

[ECode16]

If a defaultGroup in AuthGroups is empty/null just skip it instead of throwing.

[datamweb]

Hi, please explain what you need for it?

[ECode16]

My users are divided into several "basic" groups whose permissions don't intersect. (Lets say its by a continent and they can only affect things on their own continent.)

[kenjis]

The default group is only the first group set by RegisterController.
If you want to set a different group, just override the addToDefaultGroup() method.