The Diff Size column made some assumptions that certain statistical data

on the diffs were present, which wasn't the case for review requests without

diffs.
This adds some bullet-proofing to this column so that we only fetch the

information if it's available, and harmlessly fall back if not.

Viewed the dashboard and saw the diff size info come up for all review

requests containing diffs, but not those that are just file aettachmnets.

Didn't see any errors in the console.