@Override
    public void close(CompletionHandler handler) {
        if (handler != null) {
            handler.completed();
        }
    }
<p>This class provides a convenient implementation of {@link ContentChannel} that does not support being written to.
If {@link #write(ByteBuffer, CompletionHandler)} is called, it throws an UnsupportedOperationException. If {@link
#close(CompletionHandler)} is called, it calls the given {@link CompletionHandler}.</p>

<p>A {@link RequestHandler}s that does not expect content can simply return the {@link #INSTANCE} of this class for
every invocation of its {@link RequestHandler#handleRequest(Request, ResponseHandler)}.</p>

@author Simon Thoresen Hult
jdisc_core/src/main/java/com/yahoo/jdisc/handler/NullContent.java