Parse error: "transfer_type" in transfers.txt can be empty

[iblis-ms]

Hi,
Based on:
https://gtfs.org/reference/static/#transferstxt

Indicates the type of connection for the specified (from_stop_id, to_stop_id) pair. Valid options are:

0 or empty - Recommended transfer point between routes.
1 - Timed transfer point between two routes. The departing vehicle is expected to wait for the arriving one and leave sufficient time for a rider to transfer between routes.
2 - Transfer requires a minimum amount of time between arrival and departure to ensure a connection. The time required to transfer is specified by min_transfer_time.
3 - Transfers are not possible between routes at the location.

Therefore, the 'transfer_type' values can be empty, so:

inline Result Feed::add_transfer(const ParsedCsvRow & row)
{
  Transfer transfer;
  try
  {
    // Required fields:
    transfer.from_stop_id = row.at("from_stop_id");
    transfer.to_stop_id = row.at("to_stop_id");
    set_field(transfer.transfer_type, row, "transfer_type", false);

    // Optional:
    set_field(transfer.min_transfer_time, row, "min_transfer_time");
  }

should be changed to:
set_field(transfer.transfer_type, row, "transfer_type");

[mesozoic-drones]

Thank you for feedback!
While choosing which type of field to set (required or not) I used Google's official GTFS reference:
https://developers.google.com/transit/gtfs/reference?csw=1

In this reference "transfer_type" is marked as required field. But it could be empty.