The setter methods in the CSV Parser class work so that the value "null" (null surrounded by double quotes), is interpreted as an empty string. However the value 'null' (null surrounded by single quotes) is converted to an explicit string with value null (i.e. the word null as a string value). So if you set the attribute to 'null' then you will get the word null as a string value, generated for that attribute.
Here's a sample csv that puts the word null as a string value into email attribute:
action, uid, givenName, sn, cn, description, email, organization, city, state, addresses:
create,SampleUser1,Sample,User,Sample User,Sample,'null',NeteAuto,"ou=NeteAuto,dc=security,dc=com",Import,LA,111 Generic address st
The above values will actually set the mail attribute to the word null as a string value:
Sample User email = null