Fix an issue with handling of null feed URIs

Summary: Ref T8658. Caught this in the logs. This value may be set to null. Handle that gracefully.

Test Plan: Will check logs.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8658

Differential Revision: https://secure.phabricator.com/D13432