A pr.Reviewer may be nil when migrating from Gitea if this is a team
request review.
We do not migrate teams therefore we cannot map these requests, but we can
migrate user requests.
Signed-off-by: Andrew Thornton <art27@cantab.net>
// Presumably this is a team review which we cannot migrate at present but we have to skip this review as otherwise the review will be mapped on to an incorrect user.