SET GLOBAL mroonga_log_file = NULL;
ERROR HY000: NULL value can't be used for mroonga_log_file
SET GLOBAL mroonga_log_file = '';
ERROR HY000: NULL value can't be used for mroonga_log_file
SHOW GLOBAL VARIABLES LIKE 'mroonga_log_file';
Variable_name	Value
mroonga_log_file	groonga.log
