Previously we assumed that all avatar services would return URLs.

However, it is possible that the service has yet to be configured for a

user (e.g., they haven't uploaded an avatar yet for the

FileUploadAvatarService) or the avatar service returns bad data. In

these cases, we now wrap the serialization in a try..except to ensure

that we don't cause exceptions that crash the resource.
Additionally, we also ensure that we serialize the avatar_urls field

as {} instead of None when there are no URLs to simplify

deserialization logic.

Ran unit tests.