MySQL Error when importing CSV with empty fields

Something like this, checking every value for '', and inserting NULL instead, may help. Change a,b,c,d for the actual name and number of rows on the table:

$ cat /tmp/test.csv 
test1, test2, test3, test4
,,,
1,2,3,4
,,,

mysql> LOAD DATA INFILE "/tmp/test.csv" INTO TABLE test.test  
          FIELDS TERMINATED BY ','  
          OPTIONALLY ENCLOSED BY '"' 
          LINES TERMINATED BY '\n' 
          IGNORE 1 ROWS 
       (@a, @b, @c, @d) 
       SET a = IF(@a = '', NULL, @a), 
           b = IF(@b = '', NULL, @b), 
           c = IF(@c = '', NULL, @c), 
           d = IF(@d = '', NULL, @d);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Deleted: 0  Skipped: 0  Warnings: 0

mysql> SELECT * FROM test.test;
+------+------+------+------+
| a    | b    | c    | d    |
+------+------+------+------+
| NULL | NULL | NULL | NULL |
|    1 |    2 |    3 |    4 |
| NULL | NULL | NULL | NULL |
+------+------+------+------+
3 rows in set (0.00 sec)

Check the syntax for LOAD DATA for more details.