bugfix/NAV-90-create-parent-stop-to-stops-map-in-gtfs-and-raise-warning-when-parent-stop-does-not-exist

[munterfi]

• Parent stop ids can be blank strings in text input.
• Adjust parser to check for this case
• Remove Nullable parameter in addStop method of GTFS builder.

[clukas1]

Changes look good, no additional comments.