Fixes null-ref when trying to start Guest session.

CarUserSwitchingDialog threw null-ref because Guest didn't have
a default avatar.

1. Adds null guard in case default avatar is not set.
2. Populates default Guest avatar correctly.

Fixes:77546931
Fixes:78588087
Test: Manual, observing the default icon when starting a guest session.
       Manual, observing the switching dialog icon without a default icon.

Change-Id: Iacbfcc65147fdbc5515c0d96eb613fb17543f6b3