Fix a HunkQuery issue where no hunks load at all

Summary:
If you create a diff with no hunks (e.g., it adds a single empty file), we never attachHunks() so we throw on getHunks().

Instead, make sure changesets get hunks attached if they expect it.

Test Plan: Created a new diff with a single empty file in it.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: zeeg, epriestley

Differential Revision: https://secure.phabricator.com/D8842