Allow vehicle_types_available to be empty

[richfab]

Problem

In this Slack thread, @hbruch has spotted a mismatch between the specs and the JSON-Schema:

• The specs say that vehicle_types_available in station_status.json is "used to model the total number of each defined vehicle type available at a station", which can be none. (reference)
• But the JSON-Schema specifies that vehicle_types_available MUST have at least one item. (reference)

This causes a problem when a station has 0 vehicle available, like in the feed sample below:

Before	After

Suggested solution

@testower suggested to remove the "minItems": 1 constraint on vehicle_types_available in https://github.com/MobilityData/gbfs-json-schema/blob/master/v3.0-RC/station_status.json#L53.

@richfab will submit a PR.