Do not output null if no vehicle number is present in GTFS

 lib/mapper.php

@@ -135,7 +135,7 @@
                $num = $gtfsTrip['num'];
                if(!is_array($data) || !isset($data['num'])) {
                    $data = [
                        'num' => $num,
                        'num' => $num ?: '',
                        'low' => 2,
                    ];
                } elseif($data['num'] != $num) {