#13649 assumed that direct avatar urls would always be libravatar urls - this leads
to NPEs if federated avatar service is disabled.
Fix#15421
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: zeripath <art27@cantab.net>