Don't try to load profile images for projects with no profile image

Summary: Ref T8631. Projects may not have a profile image PHID; don't try to load it if they don't.

Test Plan: Saw WHERE phid IN ('') query stop happening on a local feed story page.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8631

Differential Revision: https://secure.phabricator.com/D13476