Put the data into a staging table and then insert to the production table using SQL code.
update table1
set field1 = NULL
where field1 = 'null'
Or if you want to do a lot of fields
update table1
set field1 = case when field1 = 'null' then Null else Field1 End
, field2 = case when field2 = 'null' then Null else Field2 End
, field3 = case when field3 = 'null' then Null else Field3 End