What is the desired functionality that you are missing?
Today Dashboard Variable Boolean Filters only allow viewers to filter by true or false. Variable Boolean Filters in Reports allow to filter for undefined / null values too.
We should consider aligning both by eventually changing the context switcher on Dashboard Filters to checkboxes, similar to Report Filters.
This ticket was migrated to github: https://github.com/camunda/camunda-optimize/issues/10855. Please use this link for any future references and continue any discussion there.