Always return an array from ArcanistWorkflow->loadProjectRepository()

Summary: Fixes T8344. Prior to D12971, this always returned array(). It may now sometimes return null. Switch the behavior to be more similar to the old behavior.

Test Plan: Inspection.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T8344

Differential Revision: https://secure.phabricator.com/D13061