The introduction of the fallback avatar service was intended to always

provide a useful service if the user had opted out of any other

configured services, but it didn't catch the case where the user didn't

have an explicit service configured and the administrator had configured

the default to be no service.
This change covers this case, only including the default as a viable

option if it's not None, and always including the fallback service.

Unit tests passed.
Verified manually that we no longer get errors with a None avatar

service if the default is None, and that instead we get the fallback

service.