Fix a crash on review requests without diffs.

Review Request #10412 — Created Feb. 16, 2019 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
0ad2db0...

Reviewers

The new commit list field made an assumption that all review requests
had diffsets associated with it, and would crash when the diffset came
back as 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.