Use the fallback avatar service if the default service is None.
Review Request #10516 — Created April 4, 2019 and submitted
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
service if the default is
None, and that instead we get the fallback