If the target column is coming as NULL value from source, You can give a default value by adding a column in expression transformation ad below.
COLUMN_NAME (The datatype and value filed will populate. Change it) '-1' or 'DEFAULT'
The value to the target column is defaulted to value of -1 or it can be mentioned as default depending on the datatype in the target table.
Either the column can be directly fetched from source table and given a default value or Add an column in the expression transformation by providing default value to link to target table.
Regards,
Jayanthi