handle cases where only one prediction exists

use with and pattern matching to handle sequence of possible errors

lib/screens/v2/widget_instance/line_map.ex

@@ -265,9 +265,6 @@ defmodule Screens.V2.WidgetInstance.LineMap do
   def serialize_scheduled_departure(_departures, _direction_id, _stops, true = _is_terminal?),
     do: nil
 
-  def serialize_scheduled_departure([] = _departures, _direction_id, _stops, _is_terminal?),
-    do: nil
-
   def serialize_scheduled_departure(departures, direction_id, stops, _is_terminal?) do
     # Number of departures with predictions (not just schedules) in this direction
     prediction_count =
@@ -282,16 +279,18 @@ defmodule Screens.V2.WidgetInstance.LineMap do
     if prediction_count < 2 do
       %{name: origin_stop_name} = Enum.at(stops, 0)
 
-      {:ok, local_time} =
-        departures
-        |> Enum.filter(fn d -> is_nil(d.prediction) end)
-        |> Enum.at(0)
-        |> Departure.time()
-        |> DateTime.shift_zone("America/New_York")
+      if departure = Enum.find(departures, &is_nil(&1.prediction)) do
+        {:ok, local_time} =
+          departure
+          |> Departure.time()
+          |> DateTime.shift_zone("America/New_York")
 
-      {:ok, timestamp} = Timex.format(local_time, "{h12}:{m}")
+        {:ok, timestamp} = Timex.format(local_time, "{h12}:{m}")
 
-      %{timestamp: timestamp, station_name: origin_stop_name}
+        %{timestamp: timestamp, station_name: origin_stop_name}
+      else
+        nil
+      end
     else
       nil
     end

test/screens/v2/widget_instance/line_map_test.exs

@@ -308,6 +308,12 @@ defmodule Screens.V2.WidgetInstance.LineMapTest do
       assert nil == LineMap.serialize_scheduled_departure([], direction_id, stops, false)
     end
 
+    test "returns nil when there are only predictions", %{stops: stops} do
+      direction_id = 1
+      d1 = %Departure{prediction: %Prediction{trip: %Trip{direction_id: direction_id}}}
+      assert nil == LineMap.serialize_scheduled_departure([d1], direction_id, stops, false)
+    end
+
     test "returns the correct origin", %{stops: stops} do
       direction_id = 1