Fix a crash on review requests without diffs.
Review Request #10412 — Created Feb. 16, 2019 and submitted
The new commit list field made an assumption that all review requests
had diffsets associated with it, and would crash when the diffset came
None. This is a simple fix that handles the cas of an empty
diffset more gracefully.
Loaded a review request that didn't have a diffset. Saw that the page
loaded without crashing.