[5.8] Fix qualified UPDATED_AT timestamps

[staudenmeir]

#26031 qualified UPDATED_AT timestamps to fix ambiguity issues.

This broke queries on models without an UPDATED_AT timestamp:

class User extends Model {
    public $timestamps = true;

    const UPDATED_AT = null;
}

User::query()->update(['name' => 'foo']);

update `users` set `name` = ?, `users`.`` = ?
                                       ^

Before, this case has been handled by Arr::add() . Now, we have to take care of it ourselves.