Setting SCNAction.TimingFunction to null does not read back as null

### Steps to Reproduce

1. Uncomment part of `ActionTest.cs`

```diff
diff --git a/tests/monotouch-test/SceneKit/ActionTest.cs b/tests/monotouch-test/SceneKit/ActionTest.cs
index 76c6b808..026cc487 100644
--- a/tests/monotouch-test/SceneKit/ActionTest.cs
+++ b/tests/monotouch-test/SceneKit/ActionTest.cs
@@ -35,9 +35,10 @@ namespace MonoTouchFixtures.SceneKit {
Assert.That (timeFunctionValue, Is.NaN, "TimingFunction assigned from TimingFunction2");
#endif
a.TimingFunction2 = null;
- Assert.Null (a.TimingFunction2, "TimingFunction2-end");
+ //
+ // Assert.Null (a.TimingFunction2, "TimingFunction2-end");
#if !XAMCORE_4_0
- Assert.Null (a.TimingFunction, "TimingFunction-end");
+ // Assert.Null (a.TimingFunction, "TimingFunction-end");
#endif
}
}
```

2. Run monotouch-test

### Expected Behavior

After setting `TimingFunction` to `null` we should read the delegate as `null`.

### Actual Behavior

It's non null

### Environment

xamarin-macios/master