Fix changeset construction special case for empty commits in pre-commit hooks
Summary: Fixes T13155. Ref T13151. A recent change (D19455) changed the return format here, but I missed this special case for empty commits.
Test Plan:
- T13155 has a good set of reproduction instructions.
- Pushed an empty commit.
- Before: bunch of warning log spew.
- After: clean logs.
Reviewers: amckinley, avivey
Reviewed By: avivey
Maniphest Tasks: T13155, T13151
Differential Revision: https://secure.phabricator.com/D19500