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