Arrayish fields should always return arrays

Summary:
Array-like fields in HerladCommitAdapter should always return things of
type array. If they return array(...) or null, then array_*() functions
sporadically break.

Test Plan:
Ran a PhabricatorRepositoryCommitHeraldWorker for a troublesome commit.
Failed before; fails no more.

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, Korvin, vdt

Maniphest Tasks: T1385

Differential Revision: https://secure.phabricator.com/D2793