fix: Have endpoint_stops/2 not crash when given an empty set of routes

lib/dotcom/alerts/endpoint_stops.ex

@@ -23,9 +23,15 @@ defmodule Dotcom.Alerts.EndpointStops do
   # stop-range per alert, and returns the non-nil ones.
   @impl Behaviour
   def endpoint_stops(alerts, route_ids) do
-    alerts
-    |> Enum.map(&endpoint_stops_for_alert(&1, route_ids))
-    |> Enum.reject(&Kernel.is_nil/1)
+    route_ids = route_ids |> Enum.reject(&Kernel.is_nil/1)
+
+    if route_ids |> Enum.empty?() do
+      []
+    else
+      alerts
+      |> Enum.map(&endpoint_stops_for_alert(&1, route_ids))
+      |> Enum.reject(&Kernel.is_nil/1)
+    end
   end
 
   # Returns a single endpoint stop-range for the alert given, or nil

test/dotcom/alerts/endpoint_stops_test.exs

@@ -14,7 +14,9 @@ defmodule Dotcom.Alerts.EndpointStopsTest do
   }
 
   setup do
-    Routes.Repo.Mock |> stub(:get, fn _ -> Route.build(:route) end)
+    Routes.Repo.Mock
+    |> stub(:get, fn route_id when is_binary(route_id) -> Route.build(:route) end)
+
     verify_on_exit!()
     :ok
   end
@@ -27,6 +29,17 @@ defmodule Dotcom.Alerts.EndpointStopsTest do
       assert EndpointStops.endpoint_stops([alert], [route_id]) == []
     end
 
+    # [nil] is what you get if you pull the route-informed-entities
+    # out of an alert that isn't associated with any route. Sometimes
+    # those route-informed-entities are what get passed back into the
+    # endpoint_stops/2, which is why [nil] is a relevant input to test
+    # here.
+    test "returns empty list if the route ID's provided are [nil]" do
+      alert = Alert.new(informed_entity: [%InformedEntity{stop: nil}])
+
+      assert EndpointStops.endpoint_stops([alert], [nil]) == []
+    end
+
     test "returns the same stop twice if an alert's informed entities includes precisely one stop" do
       # Setup
       route_id = FactoryHelpers.build(:id)