Prevent notice for missing configuration in PhabricatorIRCDifferentialNotificationHandler

Summary: Provide array() default so we don't foreach() over null in the case of a missing config (from @dctrwatson).

Test Plan: Will verify with @dctrwatson.

Reviewers: vrana, btrahan, dctrwatson

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3686