Add unit tests for passing null to addCallback.

src/test/java/com/spotify/futures/FuturesExtraTest.java

@@ -285,6 +285,17 @@ public void testCallbackForSuccess() throws Exception {
     verify(failure, never()).accept(any(Throwable.class));
   }
 
+  @Test
+  public void testCallbackForSuccessNullFailure() throws Exception {
+    final SettableFuture<Integer> future = SettableFuture.create();
+    final Consumer<Integer> success = mock(Consumer.class);
+
+    FuturesExtra.addCallback(future, success, null);
+
+    future.set(10);
+    verify(success).accept(10);
+  }
+
   @Test
   public void testCallbackForFailure() throws Exception {
     final SettableFuture<Integer> future = SettableFuture.create();
@@ -299,6 +310,24 @@ public void testCallbackForFailure() throws Exception {
     verify(success, never()).accept(anyInt());
   }
 
+  @Test
+  public void testCallbackForFailureNullSuccess() throws Exception {
+    final SettableFuture<Integer> future = SettableFuture.create();
+    final Consumer<Throwable> failure = mock(Consumer.class);
+
+    FuturesExtra.addCallback(future, null, failure);
+
+    final Throwable expected = new RuntimeException("boom");
+    future.setException(expected);
+    verify(failure).accept(expected);
+  }
+
+  @Test(expected = NullPointerException.class)
+  public void testCallbackWithNulls() throws Exception {
+    final SettableFuture<Integer> future = SettableFuture.create();
+    FuturesExtra.addCallback(future, null, null);
+  }
+
   @Test
   public void testSuccessCallback() throws Exception {
     final SettableFuture<Integer> future = SettableFuture.create();