Update deprecated implicitly nullable types

[tom-on-the-internet]

These implicit nullable types cause warnings.

See: https://wiki.php.net/rfc/deprecate-implicitly-nullable-types

Example:

> DB::select('SELECT datetime("now") AS current_time');

   DEPRECATED  Libsql\PDO::__construct(): Implicitly marking parameter $dsn as nullable is deprecated, the explicit nullable type must be used instead in vendor/turso/libsql/src/PDO.php on line 12.