Hey Sean -
Dealing with NULL values is a "value-add" that Desktop gives you. You have to code for it with the API. The csv2TDE.py sample code shows you how do deal with this:
for field in line:
if( colTypes[colNo] != Type.UNICODE_STRING and field == "" ) :
row.setNull( colNo )
fieldSetterMap[colTypes[colNo]](row, colNo, field);
colNo += 1
Hope this helps!