The recent work to centralize how the status line for a review request

is computed introduced a regression when viewing the diff viewer for a

brand new review request. If there's a draft diff but no existing

published diffs, we'd attempt to look up an attribute on a None

diffset, causing a page crash.
We now build information on the latest published diff only if there is

one.

Successfully opened the diff viewer without a crash on a brand new

review request.