appview/db/pulls: fix PR existence check

empty slice/array with 0 length can not be nil, check with length
instead

Signed-off-by: Seongmin Lee <[e-mail]>

appview/db/pulls.go

···
 	if err != nil {
 		return nil, err
 	}
-	if pulls == nil {
+	if len(pulls) == 0 {
 		return nil, sql.ErrNoRows
 	}