How to handle null in Datamodel

[#1]

Dear All

I have a datamodel, when i select the different list box like type,trnnum,channel it shows the null in the list box how i remove this, and is null values can effect my calculation in text box.

LOAD 'IT' as Flag,
     '' as Type,
     '' as [Cheque no],
     '' as Channel,
     '' as TRNNUM,
     ACCNUM,
     BNFACCNUM,
     FUNTRAAMT as AMOUNT,
     COMM as COMMISION
    FROM
[....qvd]
(qvd);
Concatenate
LOAD 'Sales' as Flag,
      '' as Type,
      '' as [Cheque no],
     Channel,
     TRNNUM,
     '' as ACCNUM,
     BNFACCNUM,
     amount as AMOUNT,
     '' as COMMISION    
   FROM
[...qvd]
(qvd);

Concatenate
LOAD 'Creditinfo' as Flag,
     Type,
     [Cheque no],
     '' as Channel,
     '' as TRNNUM,
     ACCNUM,
     BNFACCNUM,
     AMOUNT,
    '' as COMMISION
     FROM
[....qvd]
(qvd);