there are several ways to get the result that you want, but take into account that a primary key cannot be null.
I show you how to do it with other fields.
The way that you specified in your email is the most difficult one. It will oblige you to separate the flow of the application in case the boolean field is true or false.
Another option is to use a Query Unit and pass the value coming from the form to it, execute the desired query and then show the results into a PowerIndexUnit, by retrieving the elements by oid.
The easiest way to do that is to create a calculated attribute on the entity, called for example "isNull", that is equal to true when the desired field is null and false in the opposite case. Then use a normal attribute condition based on the calculated attribute.