How to create Empty IQ Packet using Smack 4

I am develloping call signals using Smack 4. I want to send an IQ-result which have the format below using Smack 4 and Java:

<iq from="sister@realworld.lit/home"
id="v73hwcx9"
to="alice@wonderland.lit/rabbithole"
type="result"/>

This is what I did and did not work

public class AcknowledgeIQ extends IQ {

    public AcknowledgeIQ() {
        super("");
        setType(Type.result);
    }

    @Override
    public IQChildElementXmlStringBuilder getIQChildElementBuilder(IQChildElementXmlStringBuilder xml) {
        return null;
    }

    public String toXml(){
        return super.toXML(null).toString();
    }
}

I have tested below code on the SMACK 4.3.4.

public class AcknowledgeIQ extends org.jivesoftware.smack.packet.IQ {

    public AcknowledgeIQ() {
        super(null, null);
    }

    @Override
    public IQChildElementXmlStringBuilder getIQChildElementBuilder(IQChildElementXmlStringBuilder xml) {
        xml.rightAngleBracket();
        return xml;
    }
}

Usage example :

AcknowledgeIQ ackIQ = new AcknowledgeIQ();
ackIQ.setType(org.jivesoftware.smack.packet.IQ.Type.result);
Log.d(TAG, "AcknowledgeIQ = "+ackIQ.toXML(null).toString());

Logcat output :

AcknowledgeIQ = <iq xmlns='jabber:client' to='alice@wonderland.lit/rabbithole' id='MGr5o-5' type='result'></iq>