When a draft review request was added without an associated diff, we

would fail to render the reviewable page model data due to assuming that

the review request details (which might be a draft or a review request)

would walways have a diffset attached. This assumption has been

corrected.

Am now able to view a review request details page for a review request

created without commit history support when the current draft does not

have a diff attached.
When there is a draft with a diffset attached, the draft commits are

shown instead.