Add in Observable.empty method that emits nothing, and test for it

library/src/main/java/com/anthonycr/bonsai/Observable.java

@@ -69,6 +69,23 @@ public static <T> Observable<T> create(@NonNull Action<T> action) {
         return new Observable<>(action);
     }
 
+    /**
+     * Static creator that creates an Observable that is empty
+     * and emits no items, but completes immediately.
+     *
+     * @param <T> the type that will be emitted to the onSubscribe
+     * @return a valid non-null empty Observable.
+     */
+    @NonNull
+    public static <T> Observable<T> empty() {
+        return new Observable<>(new Action<T>() {
+            @Override
+            public void onSubscribe(@NonNull Subscriber<T> subscriber) {
+                subscriber.onComplete();
+            }
+        });
+    }
+
     /**
      * Tells the Observable what Scheduler that the onSubscribe
      * work should run on.

library/src/test/java/com/anthonycr/bonsai/ObservableUnitTest.java

@@ -39,7 +39,7 @@
 /**
  * To work on unit tests, switch the Test Artifact in the Build Variants view.
  */
-public class ObservableUnitTest extends BaseUnitTest{
+public class ObservableUnitTest extends BaseUnitTest {
 
     @Test
     public void testMainLooperWorking() throws Exception {
@@ -402,4 +402,26 @@ public void onNext(@Nullable String item) {
         Assert.assertTrue("isUnsubscribed() was not correct", unsubscribed.get());
     }
 
+    @Test
+    public void testObservableEmpty_emitsNothingImmediately() throws Exception {
+        final Assertion<Boolean> onNextAssertion = new Assertion<>(false);
+        final Assertion<Boolean> onCompleteAssertion = new Assertion<>(false);
+        Observable.empty().subscribe(new OnSubscribe<Object>() {
+
+            @Override
+            public void onNext(@Nullable Object item) {
+                onNextAssertion.set(true);
+            }
+
+            @Override
+            public void onComplete() {
+                onCompleteAssertion.set(true);
+            }
+
+        });
+
+        Assert.assertFalse(onNextAssertion.get());
+        Assert.assertTrue(onCompleteAssertion.get());
+    }
+
 }