[Chromecast] Add test case for Observable#map().

reactive_java.md documents the expectation that the Observable
produced by map() is not activated when its source is activated
and the Function that transforms the activation data returns
null, but this expectation was not properly tested.

Bug: None
Test: cast_base_junit_tests
Change-Id: If8ff1764efe6790a9eeabcfeadf28b052ce924f5
Reviewed-on: https://chromium-review.googlesource.com/1120643Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572289}

chromecast/base/java/test/org/chromium/chromecast/base/ObservableAndControllerTest.java

@@ -362,6 +362,15 @@ public class ObservableAndControllerTest {
                         "exit upper"));
     }
 
+    @Test
+    public void testMapDropsNullResult() {
+        Controller<Unit> controller = new Controller<>();
+        ReactiveRecorder recorder = ReactiveRecorder.record(controller.map(x -> null));
+        controller.set(Unit.unit());
+        // Recorder should not get any events because the map function returned null.
+        recorder.verify().end();
+    }
+
     @Test
     public void testFilter() {
         Controller<String> a = new Controller<>();