How to set NULL value by arcpy.da.InsertCursor on date column?

How to set NULL value by arcpy.da.InsertCursor on date column?

I try insert "NULL", None, '', 0

dictIN= [{'field1':"test1",'date1':'01-02-2015','date1':''},\
 {'field1':"test2",'date1':'01-02-2015','date1':'01-02-2015'},\
 {'field1':"test3",'date1':'','date1':''},\
 {'field1':"test4",'date1':'01-02-2015','date1':''}]


def setNULLIf(field):
    if field == '':
        return None #try NULL,'',0
    else: return field

cursor = arcpy.da.InsertCursor(GDB,("field1", "date1", "date2"))
for x in dictIN:
    cursor.insertRow((x[field1'], setNULLIf(x['date1']), setNULLIf(x['date2'])))

Is it possible by arcpy.da ? I use ArcGIS 10.2. I try insert data to GDB file.