title: Filter for values OR IS NULL in activeadmin
body: <p>Given I have two models:</p>
  
  <pre><code>class Post &lt; ActiveRecord::Base
    belongs_to :district
  end
  
  class District &lt; ActiveRecord::Base
    has_many :posts
  end
  </code></pre>
  
  <p>I need to make a <code>check_boxes</code> filter in ActiveAdmin on Posts page for with ability for user to get posts that belong to some exact districts or does not belong to any districts at all.</p>
  
  <p>Before ActiveAdmin changed MetaSearch to Ransack, that could be done with custom scope. And now I don't have any idea. When I do the following:</p>
  
  <pre><code>filter :district_id_null, as: :boolean
  filter :district, as: :check_boxes
  </code></pre>
  
  <p>It makes condition <code>WHERE district_id IN (1,2,3) AND district_id IS NULL</code> (I need OR instead of AND). And when I do </p>
  
  <pre><code>filter :district, as: :check_boxes, collection: proc { District.unscoped.map { |x| [x.title, x.id] }.unshift ['Empty', 'null'] }
  </code></pre>
  
  <p>It makes condition <code>WHERE district_id IN (0,1,2,3)</code> (but in most SQL databases NULL is not 0).</p>