Gorm Inconsistent default value check between MySQL and MariaDB

Consider the following schema:

type User {
    ID string `gorm:"primary_key"`
    Name string `gorm:"unique"`
}

Note that no explicit default value was set for a user's name column.

Moreover, consider the following part of the auto migration functionality:

https://github.com/go-gorm/gorm/blob/19b8d37ae8155667d76021e4ca3314bb571756be/migrator/migrator.go#L451-L456

field.DefaultValue is always "", however, columnType.DefaultValue() returns "NULL" on MariaDB and "" on MySQL, as a consequence of which an ALTER TABLE query will be issued on MariaDB although nothing had actually changed. In combination with https://github.com/go-gorm/mysql/issues/74 and the fact that only 64 keys per table are allowed on MariaDB, this can cause a program to crash over time.

A similar issue was already brought up in https://github.com/go-gorm/gorm/issues/5188, I think.

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