private static SubscriptionImpl noopSubscription() {
        return new SubscriptionImpl(NoopCancellableStreamMessage.INSTANCE, NoopSubscriber.get(),
                                    ImmediateEventExecutor.INSTANCE, EMPTY_OPTIONS);
    }
Closes the deferred stream without setting a delegate.

@throws IllegalStateException if the delegate has been set already or
                              if {@link #close()} or {@link #close(Throwable)} was called already.
core/src/main/java/com/linecorp/armeria/common/stream/DeferredStreamMessage.java