Can someone please give me a hint how to prevent a question mark for a blanc date in an export to .txt-file?

The txt-file I create is suppost to have a blanc field when a date is not filled. In my ar_mstr the field ar_paid_date is sometimes empty. On exporting the ar_mstr-record, in the export file appears a "?" in stead of a blanc field between my delimiters. So :

ar_type|ar_nbr|ar_cust|ar_so_nbr|ar_amt|ar_ship|ar_open|ar_dun_level|ar_due_date|ar_paid_date
"I"|"SI337747"|"606124"|"MA000264"|-68,6|"606124BE"|yes|0|14/03/16|?
"I"|"SI337860"|"229008"|"SO332579"|28747,8|"229008GE"|yes|0|31/01/16|01/02/16
 

in stead of

ar_type|ar_nbr|ar_cust|ar_so_nbr|ar_amt|ar_ship|ar_open|ar_dun_level|ar_due_date|ar_paid_date
"I"|"SI337747"|"606124"|"MA000264"|-68,6|"606124BE"|yes|0|14/03/16|
"I"|"SI337860"|"229008"|"SO332579"|28747,8|"229008GE"|yes|0|31/01/16|01/02/16
 

Many thanks in advance