Change assertions to be more inline with java.util.concurrent.ThreadPoolExecutor

The current assertions do not allow you to set the queue capacity to zero to use the SynchronousQueue set by the below condition

```java
if (queueCapacity == 0) {
    blockingQueue = new SynchronousQueue<Runnable>();
} else {
    blockingQueue = new LinkedBlockingQueue<Runnable>(queueCapacity);
}
```