public String XMLString {get;set;}
private list<LightiningEd__Webinar_Attendees_Status__c> lstevents{get;set;}
LightiningEd__Webinar_Attendees_Status__c adobeevent;

global XMLparserUpdate(){
    lstevents = new list<LightiningEd__Webinar_Attendees_Status__c>();

String 1: customized response inserts data perfectly     //XMLString ='<?xml version="1.0" encoding="utf-8"?><user_list><user principal_id="1304153011" permission_id="View" name="Test T" login="luckchan12@gmail.com" registration_time="12/21/2017 4:09 AM" attendance_status="Not Attended" first_in_time=" " duration="00:00:00" isLead="No" registration_question_1304085214=" " campaign_alias_1304085214=" " idp-source=" "/><user principal_id="1300650026" permission_id="View" name="chanbasha sk" login="skbasha467@gmail.com" registration_time="12/21/2017 3:51 AM" attendance_status="Attended" first_in_time=" " duration="01:23:44" isLead="No" registration_question_1304085214=" " campaign_alias_1304085214=" " idp-source=" "/><user principal_id="1304198970" permission_id="View" name="Basha Sk" login="basha183755@gmail.com" registration_time="12/21/2017 4:19 AM" attendance_status="Not Attended" first_in_time=" " duration="00:00:00" isLead="No" registration_question_1304085214=" " campaign_alias_1304085214=" " idp-source=" "/><user principal_id="1304208474" permission_id="View" name="Testt tt" login="sr7041046@gmail.com" registration_time="12/21/2017 4:17 AM" attendance_status="Not Attended" first_in_time=" " duration="00:00:00" isLead="No" registration_question_1304085214=" " campaign_alias_1304085214=" " idp-source=" "/></user_list>';
String 2: actual response inserts null values            XMLString ='<?xml version="1.0" encoding="utf-8"?><results><status code="ok"/><poll_questions/><registration_questions><question id="1304085214" description="Campaign Tracking ID"/></registration_questions><user_list><user principal_id="1304153011" permission_id="View" name="Test T" login="luckchan12@gmail.com" registration_time="12/21/2017 4:09 AM" attendance_status="Not Attended" first_in_time=" " duration="00:00:00" isLead="No" registration_question_1304085214=" " campaign_alias_1304085214=" " idp-source=" "/><user principal_id="1300650026" permission_id="View" name="chanbasha sk" login="skbasha467@gmail.com" registration_time="12/21/2017 3:51 AM" attendance_status="Attended" first_in_time=" " duration="01:23:44" isLead="No" registration_question_1304085214=" " campaign_alias_1304085214=" " idp-source=" "/><user principal_id="1304198970" permission_id="View" name="Basha Sk" login="basha183755@gmail.com" registration_time="12/21/2017 4:19 AM" attendance_status="Not Attended" first_in_time=" " duration="00:00:00" isLead="No" registration_question_1304085214=" " campaign_alias_1304085214=" " idp-source=" "/><user principal_id="1304208474" permission_id="View" name="Testt tt" login="sr7041046@gmail.com" registration_time="12/21/2017 4:17 AM" attendance_status="Not Attended" first_in_time=" " duration="00:00:00" isLead="No" registration_question_1304085214=" " campaign_alias_1304085214=" " idp-source=" "/></user_list></results>';

    DOM.Document doc=new DOM.Document();
    try{
        doc.load(XMLString);
        DOM.XmlNode userList=doc.getRootElement();
        for(Dom.XmlNode userNode : userList.getChildElements()){

            System.debug(userNode.getAttributeValue('principal_id', null) + ' : ' + userNode.getAttributeValue('name', null) + ' : ' + userNode.getAttributeValue('attendance_status', null));

            adobeevent = new LightiningEd__Webinar_Attendees_Status__c();

            adobeevent.name = userNode.getAttributeValue('name', null);
            adobeevent.LightiningEd__Login__c = userNode.getAttributeValue('login', null);
            adobeevent.LightiningEd__Attandance_Status__c = userNode.getAttributeValue('attendance_status', null);
            adobeevent.LightiningEd__principal_id__c = userNode.getAttributeValue('principal_id',null);
            lstevents.add(adobeevent);
            System.debug('lstevents:::'+lstevents);

        }
    }catch(Exception e){

        System.debug(e.getMessage());
    }
    if(lstevents.size()>0){
        upsert lstevents;
    }
}