Fix an initialization issue in VectorTree
Summary: Ref T13520. In unusual cases where there are no changes in a changeset list (e.g., empty commits) we can fatal when trying to iterate over an empty list of vectors.
- Created an empty commit.
- Used "git show | pbcopy" to create a diff from it.
- Viewed it in the web UI.
- Before: fatal when iterating on null.
- After: clean page.
Maniphest Tasks: T13520
Differential Revision: https://secure.phabricator.com/D21221