mysqlimport imports NULL values unwantedly as int 0
This issue is caused by the way MySQL LOAD DATA handles NULLs:


  Handling of NULL values varies according to the FIELDS and LINES
  options in use:
  
  For the default FIELDS and LINES values, NULL is written as a field
  value of \N for output, and a field value of \N is read as NULL for
  input (assuming that the ESCAPED BY character is ).
  
  If FIELDS ENCLOSED BY is not empty, a field containing the literal
  word NULL as its value is read as a NULL value. This differs from the
  word NULL enclosed within FIELDS ENCLOSED BY characters, which is read
  as the string 'NULL'.
  
  If FIELDS ESCAPED BY is empty, NULL is written as the word NULL.
  
  With fixed-row format (which is used when FIELDS TERMINATED BY and
  FIELDS ENCLOSED BY are both empty), NULL is written as an empty
  string. This causes both NULL values and empty strings in the table to
  be indistinguishable when written to the file because both are written
  as empty strings. If you need to be able to tell the two apart when
  reading the file back in, you should not use fixed-row format.


In your case NULL is considered as word 'NULL' and if you try to cast 'NULL' to number it will return 0. You need to change NULL to \N or enclose strings using some character.