Fix creating zero length char with MySQL

Fix creating zero length char with MySQL

https://dev.mysql.com/doc/refman/8.0/en/string-type-syntax.html
> MySQL permits you to create a column of type CHAR(0). This is useful primarily when you must be compliant with old applications that depend on the existence of a column but that do not actually use its value. CHAR(0) is also quite nice when you need a column that can take only two values: A column that is defined as CHAR(0) NULL occupies only one bit and can take only the values NULL and  (the empty string).