In Oracle, if you know that a particular table column may contain a null value, then you should use NVL() function to compare it. For example:

select * from emp
   where nvl(comm, 0) = 100;

If a field type is varchar2, check the following example:

select * from product_Info
   where nvl(product_tags, 'x') = 'BEST SELLER';

In Oracle Apex, you want to filter the report result based on a page item for a particular column; check the following example for it:

select item_id, item_name, cost_price, selling_price 
   from item_master
   where nvl(category, 'x') = nvl(:p2_category, nvl(category, 'x'));

If the user has specified the category value for page item p2_category, it will return the rows only matches with the p2_category, and if the user didn’t select the value for p2_category, meaning p2_category is null, then it will return all rows.

Also, you need to specify the page item p2_category in items to submit property in Oracle Apex.