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.

Test Plan:

• 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