I am trying to get the records from database which contains `parent_item_id` **NULL** value.

```
$order_ = $this->_objectManager->create('Vendor\Module\Model\Saleslist')
                    ->getCollection()
            ->addFieldToFilter(
                'order_id',
                $orderId
            )->addFieldToFilter(
                'parent_item_id',
                ['eq' => NULL]
            );

```

and also want to add the `parent_item_id` equal **NULL** or **' '** string check.