Prevent NPE whilst migrating if there is a team request review (#19855)

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>

services/migrations/gitea_uploader.go

@@ -696,6 +696,8 @@ func convertReviewState(state string) models.ReviewType {
 		return models.ReviewTypeReject
 	case base.ReviewStateCommented:
 		return models.ReviewTypeComment
+	case base.ReviewStateRequestReview:
+		return models.ReviewTypeRequest
 	default:
 		return models.ReviewTypePending
 	}