R1:6136b8327575

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.