Update modify_attr_write2db.php

In OpenSuSE 13.1, when update/add any attribute, because the mysql database  "max_length" field is integer
example:
UPDATE ConfigAttrs SET attr_name = 'register', friendly_name = 'register', description = '', max_length = '', poss_values = '0::1', predef_value = '0', mandatory = 'yes', ordering = '26', visible = 'yes', write_to_conf = 'yes' WHERE id_attr = 231
error:
mysql error:Incorrect integer value: '' for column 'max_length' at row 1