如何添加按钮或链接到YII2 Gridview :: FILTER_DATE

I am trying to add button or link to the filter_date popup calendar to allow filtering gridview not only by particular date but also to display null values only (like excel autofilter empty values).

On Search model i setup query : ($this->delivery_date)=="null" ? $query->andWhere(['delivery_date' => null,]) : $query->andFilterWhere(['delivery_date' => $this->delivery_date]);

Now i need to allow in search field 'null' value apart of date. Is there a simple way to modify calendar widget ?