Don't throw an exception if no configuration is set for linter tests

Summary: This was broken in D9641. If `$config` is not set, then a `PhutilJSONParserException` will be thrown. The expected behaviour is that it is okay to not explicitly set any configuration.

Test Plan: Ran `arc unit`.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9643