Need way to make filter fields optional

I love the approach of using FILTER_ATTRIBUTES - but filtering should be optional

e.g. 

  FILTER_ATTRIBUTES = [
    :paid,
    :album_status
  ].freeze
  
  paid is a boolean
  
  most of the time - I don't want to filter on paid. So the filter option needs three states:

-   don't mind
-   true
-   false 
