The Facility class requires `stop_id` to be populated in `facilities.txt`

[Emma Simon]

Summary:

When trying to load the MBTA GTFS files, which include the facilities extension, an exception is thrown because some entries have a blank stop_id field.

Steps to reproduce:

Download the MBTA GTFS file and try to read it with a GtfsReader.

https://www.mbta.com/developers/gtfs

Expected behavior:

The GTFS file is loaded successfully.

Observed behavior:

An exception is thrown when the required stop_id field is empty.

Platform:

MacOS, Java 17, onebusaway-gtfs-modules 1.3.119