Importing CSV with NULL values to a Table

[kenneth dyson]

I am using the "Table Data Import Wizard" to input a csv file containing columns of number data.
Some cells are NULL, and have in them the characters NULL.
When I start the wizard all the "Field Types" are given as "text", and I can see the NULL entries in the preview pane.
I then change them to "int" or "double". and
I have the "Encoding" at utf-8.
I then run through the rest of the wizard.
The imported data no longer has the NULL data, all the rows with any NULL values were dropped.

If I import with all the "Fields Types" set to "text" I get all the rows, but then I can't treat the values as numbers for operations in MySQL, correct?

I tried changing the "Field Types" to "int" or "double" after importing as "text" but was greeted with errors:
ALTER TABLE tablename MODIFY columnname INTEGER;
Error Code: 1265. Data truncated for column 'columnname' at row 1 0.024 sec

Any ideas as to what I should do?
Thanks