Yii2: save checkbox false as NULL

In Yii2 I have ActiveForm with checkbox field. In mysql database it is tinyint(1) column that can be NULL.

I need unchecked checkbox (false value) to be saved as NULL in database. Currently when checkbox is unchecked, it is saved as (int) 0.

What is the proper way to save false value as NULL?

Here is the model:

class ProductToProductCategory extends ActiveRecord {

    public function rules()
    {
        return [
            ['product_category_id', 'required'],
            ['product_category_id', 'integer'],
            ['is_main', 'boolean'],
        ];
    }
}

Here is the view:

<?= $form->field($model, "is_main")->checkbox() ?>