Why? Currently for a checkbox type custom field, it is only possible to filter by EQUALS to YES or NO
Sometimes it is required to Filter the Null values, eg: Create a list as a check to see if this field is missed during input.
What? : Filter option to include "Null" in the list, or an option "Not Equal to" or "Empty and Not Empty".
Note: This should include filter in custom views as well as Notifications