fix(UpcomingDepartures): Don't return nil departure_seconds when prediction has nil times

[joshlarson]

Scope

Asana Ticket: ArithmeticError: bad argument in arithmetic expression

In the rare cases when a prediction sets arrival and departure time to nil, but doesn't have a schedule_relationship of :skipped or :cancelled, then UpcomingDepartures.upcoming_departures/1 will return an upcoming departure with an arrival status of {:departure_seconds, nil}, which then causes an ArithmeticError when ScheduleFinderLive tries to render it.

Implementation

• Handle the case when a prediction sets arrival time and departure time to nil.
• I made the assumption that we'd want to render those the same way we render "Scheduled" departures, and that we'd want to drop them for subway.

How to test

This is an unusual situation, one that I've never seen in the wild. I was only able to validate this by running the test cases added in this PR.

[lvachon1]

Looks good from here