Remove some unnecessary NULL checks.

* When removing a buddy, the group is optional as noted in the description.
* When setting a buddy icon, the icon may be %NULL to remove existing icons.

Testing Done:
Compile and watch for assertion failures in the log, when signing out of Bonjour, and when removing a buddy icon from an account (which did nothing.)

Reviewed at https://reviews.imfreedom.org/r/445/