for i := 0 to FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes.Count - 1 do
begin
            strCreate_Date:=FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes[i].ChildValues['CREATE_DATE'];

//if (FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes[i].NodeName='DELIVERY_DATE') then ...
if (FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes.FindNode('DELIVERY_DATE')<>Nil) then
strDelivery_Date:=FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes[i].ChildValues['DELIVERY_DATE'];
else ;

CDSGetBuyerWaybills.Append;
CDSGetBuyerWaybills.FieldByName('create_date').AsString:=strCreate_Date;
CDSGetBuyerWaybills.FieldByName('delivery_date').AsString:=strDelivery_Date;
CDSGetBuyerWaybills.FieldByName('activate_date').AsString:=strActivate_Date;
CDSGetBuyerWaybills.Post;
end;

if (FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes.FindNode('DELIVERY_DATE')<>Nil) then...

if (FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes[i].NodeName='DELIVERY_DATE') then ...

<?xml version="1.0"?>

<WAYBILL_LIST xmlns="">
    <WAYBILL>
         <ID>206405566</ID>
         <TYPE>2</TYPE>
         <CREATE_DATE>2015-09-29T12:10:53</CREATE_DATE>
         <TRANSPORT_COAST>0</TRANSPORT_COAST>
         <DELIVERY_DATE>2015-10-03T10:34:43</DELIVERY_DATE>
         <STATUS>2</STATUS>
         <ACTIVATE_DATE>2015-09-29T12:10:55</ACTIVATE_DATE>
         <FULL_AMOUNT>279.99</FULL_AMOUNT>
         <CAR_NUMBER>UJU991</CAR_NUMBER>
         <WAYBILL_NUMBER>0198315109</WAYBILL_NUMBER>
         <CLOSE_DATE>2015-10-03T10:34:43</CLOSE_DATE>
         <S_USER_ID>26461</S_USER_ID>
         <BEGIN_DATE>2015-09-29T12:10:53</BEGIN_DATE>
   </WAYBILL>
   <WAYBILL>
         <ID>207247983</ID>
         <TYPE>2</TYPE>
         <CREATE_DATE>2015-10-03T12:20:09</CREATE_DATE>
         <TRANSPORT_COAST>0</TRANSPORT_COAST>
         <STATUS>1</STATUS>
         <ACTIVATE_DATE>2015-10-03T12:20:11</ACTIVATE_DATE>
         <FULL_AMOUNT>279.56</FULL_AMOUNT>
         <CAR_NUMBER>YPY088</CAR_NUMBER>
         <WAYBILL_NUMBER>0199129044</WAYBILL_NUMBER>
         <S_USER_ID>26461</S_USER_ID>
         <BEGIN_DATE>2015-10-03T12:20:09</BEGIN_DATE>
    </WAYBILL>
</WAYBILL_LIST>

If Pos('<DELIVERY_DATE>', XmlResponse) > 0 then // значит тэг есть.
...
else // тэга нет.

with FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes[i] do
    //сначала обязательно проверим наличие элемента
    //чтобы потом не попытаться обратиться к неициализированному объекту и не вызвать AV
    if ChildNodes.FindNode('DELIVERY_DATE') = nil then //элемента не существует
        ShowMessage('There is no DELIVERY_DATE element in this Waybill!')
    else //элемент существует
    begin
      ...
    end;

if (FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes[i].ChildValues['DELIVERY_DATE']<>Null)
    then
    strDelivery_Date:=FromServ.get_buyer_waybillsResult.XMLNode.ChildNodes[i].ChildValues['DELIVERY_DATE']
    else;