title: Ransack not to filter issue
body: <p>I have a filter in ActiveAdmin resource page:</p>
  
  <pre><code>  filter :active_in, label: 'Active', as: :select, collection: ['All', 'Yes', 'No']
  </code></pre>
  
  <p>and ransacker for it:</p>
  
  <pre><code>  ransacker :active, formatter: proc {|value|
      results = Model.active.ids if value == 'Yes'
      results = Model.inactive.ids if value == 'No'
      results.present? ? results : nil
  
    } do |parent|
      parent.table[:id]
    end
  </code></pre>
  
  <p>The issue is if i select 'All' it should display all the data without filtering, instead if this it tries to filter Model's by id with 'null'.</p>
  
  <pre><code>SELECT COUNT(count_column) FROM (SELECT  1 AS count_column FROM `models` WHERE `models`.`id` IN (NULL) LIMIT 50 OFFSET 0) subquery_for_count
  </code></pre>
  
  <p>For 'Yes' and 'No' it works correctly.</p>