fix(ScheduleFinderLive): handle empty departures

[thecristen]

Scope

Found this bug after I did some load testing; it's an edge case which happens when going outbound from a terminus (e.g. Southbound from the southernmost stop), which is a situation which has no departures.

** (BadMapError) expected a map, got:
   nil
   (elixir 1.19.5) lib/map.ex:541: Map.get(nil, :time, nil)
   (dotcom 0.0.1) lib/dotcom_web/live/schedule_finder_live.ex:363: anonymous fn/4 in DotcomWeb.ScheduleFinderLive.assign_last_trip_time/1
   (phoenix_live_view 1.1.26) lib/phoenix_live_view/async.ex:141: anonymous fn/2 in Phoenix.LiveView.Async.assign_async/4
   (phoenix_live_view 1.1.26) lib/phoenix_live_view/async.ex:289: Phoenix.LiveView.Async.do_async/5
   (elixir 1.19.5) lib/task/supervised.ex:105: Task.Supervised.invoke_mfa/2
   Function: #Function<7.55729389/0 in Phoenix.LiveView.Async.run_async_task/

Implementation

I gave it a map like it wanted.

How to test

Was easy to find this bug at the "wrong" direction from termini, e.g. Green Line towards Riverside from the Riverside stop.

[jlucytan]

lgtm, great catch!