Query Criteria If On Form Field Limit, Otherwise Do Nothing?

I tried to word the title as accurately as possible.
What I'm trying to build is a Form where they can select data in different fields to limit (filter) the list. In my query criteria I want to read the form, if there's a value use THAT in the criteria (for that field) otherwise ignore it.

I've tried isnulls etc. If I put a value (like City) it properly returns all the records with that city, however if I put nothing the "ISNULL" returns a value that the criteria doesn't match i.e. I get nothing.

There must be a way to tell a query to NOT use any criteria if an object (form!field) is blank...?