Made password optional when calling UpdateUserAuth.

Updating a UserAuth without having the raw password is not possible.  

Made the password parameter optional in the `UpdateUserAuth` method.  

I also believe that this was your original intention that this should be possible seeing the null checks happening on the password variable within `UpdateUserAuth`. These are useless in the current implementation because an exception will be thrown on the assert in `ValidateNewUser` and the code will never reach the null checks anyway.