Magento 2: How create ui grid filter for null values in specific column

Question

I have a column "purchase_point" in my table. It has 3 values android, ios, and null. I need to display it in the ui grid. For the null value, I need to display 'web'. I am using the below query to get the value and it is working fine in the grid column. But the filter is not working.Please help me to resolve this

 <column name="purchase_point">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                 <item name="filter" xsi:type="string">text</item>
                <item name="label" xsi:type="string" translate="true">Order Place Point</item>
                </item>
            </argument>
        </column>

This is the query to return the field value for the purchasepoint

 $collection->getSelect()->joinLeft(
                    ['so' => 'sales_order'],
                    "main_table.order_id = so.entity_id",
                    ''
                )->joinLeft(
                    ['mop'=>'mobikul_orderPurchasePoint'],
                    "so.increment_id = mop.increment_id",
                    ['purchase_point' => 'IF(mop.purchase_point IS NOT NULL, mop.purchase_point, "Web")']
                )->group(["main_table.order_id","main_table.product_id"]);