Want to Retain NULL values while importing Data From OLEDB Source(table) to Flat text file

I'm generating flat Pipe delimited text file from a SQL OLEDB(database table) using SSIS DATA FLOW TASK.

Some Columns contain blank or some of the column contain NULL values . WHen file is genearting it showing Blank('  ') in place of NULL/blank.

e.g. data is showing like ABC||XYZ| but I need the format likeABC|NULL|XYZ|NULL.

Please help me in this regard.

Anu