You would have to change this manually in the code.
The column type array is built with three sections... the third part is the value to insert when blank.... it will look something like:
$WA_columnTypesStr = "',none,''|',none,''|',none,''|none,none,NULL|...
the column types are separated by | and there will be one for each column referenced above like:
$WA_fieldNamesStr = "Column1|Column2|Column3|Column4|...
If I wanted Column2 to insert null when blank I would update it to:
$WA_columnTypesStr = "',none,''|',none,NULL|',none,''|none,none,NULL|...