Add '??' prefix when using GTFS vehicle ID

 lib/mapper.php

@@ -135,7 +135,7 @@
                $num = $gtfsTrip['num'];
                if(!is_array($data) || !isset($data['num'])) {
                    $data = [
                        'num' => $num ?: $gtfsTrip['id'],
                        'num' => $num ?: '??'.$gtfsTrip['id'],
                        'low' => NULL,
                    ];
                } elseif($data['num'] != $num) {