Java (jsii): passing "null" behaves differently than no argument

For example:

```java
// no exception
new Queue(stack, "foo");

// exception: org.jsii.JsiiException: Cannot read property 'deliveryDelaySec' of null
new Queue(stack, "foo", null);

// workaround:
new Queue(stack, "foo", QueueProps.builder().build());
```
