I faced this issue multiple times already. You can work around this issue but I think it should be a part of the ORM.

Table:

CREATE TABLE categories
(
    id INTEGER PRIMARY KEY NOT NULL,
    name VARCHAR(128) NOT NULL,
    meta_title VARCHAR(255)
);

ORM usage:

$entity = $this->Categories->newEntity($this->request->data);
$this->Categories->save($entity);

Current result:

Should be:

As I mentioned above you can work around this behavior with Behaviors/Callbacks but I think this should be something which is handled directly inside the ORM. Saving an empty string inside a NULLable field isn't the proper usage of NULL.