add test case for null next stop

test/resolvers/OrderedStopResolverTests.test.ts

@@ -107,7 +107,26 @@ describe("OrderedStopResolvers", () => {
     });
 
     it("returns null if there is no next stop in the repository", async () => {
+      const orderedStops = await setUpOrderedStopsInRepository();
+      orderedStops[0].nextStop = undefined;
+      await repository.addOrUpdateOrderedStop(orderedStops[0]);
 
+      const response = await testServer.executeOperation({
+        query,
+        variables: {
+          systemId: mockSystem.id,
+          routeId: mockRoute.id,
+          stopId: orderedStops[0].stopId,
+        },
+      }, {
+        contextValue: {
+          repository,
+        }
+      });
+
+      assert(response.body.kind === "single");
+      const nonexistentNextStop = (response.body.singleResult.data?.system as any).route.orderedStop.nextStop;
+      expect(nonexistentNextStop).toBeNull();
     });
 
     it("returns null if the current stop no longer exists", async () => {