According to the documentation, you can use
SET statements to transform the data on the way in.
[SET col_name = expr,...]
The expr expression can include the column name, which will be interpreted as the data being read from the file and destined for that column... so, for example, at the end of your
LOAD DATA INFILE statement you might use:
SET latitude = IF(latitude + 0 = 0,NULL,latitude),
area_code = IF(area_code = '',NULL,area_code)
This example transforms 2 columns. If latitude + 0 is 0, latitude gets set to
NULL, and otherwise it gets set to the value from the file as the data is inserted; if area_code contains an empty string, it gets set to
NULL, otherwise to the data from the file. The more appropriate choice will depend on how MySQL handles casting the data, but I suspect either of these constructs would work in your situation.
You do not have to reference columns you don't intend to transform. They'll be inserted as-is.