Allow DifferentialDiff to construct proper DifferentialChangeset objects from diffs which add empty files

Summary:
See T507 and some others. We now parse empty git diffs correctly, but the logic
to build DifferentialDiffs out of them leaves the objects with 'null' for
$changesets, when it should be array().

Further layers later throw, believing we have not loaded the changesets, when we
actually have, there just aren't any.

Test Plan: Viewed rJX05d493e17fbbb29f29e4880be6834d1d7415374e in Diffusion,
which adds an empty README file. No exception thrown.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: nh

CC: aran, nh

Differential Revision: 1038