title: How to apply custom sort to column in AcitveAdmin
body: <p>I have a Movie model with a name and release_date attribute, and the following ActiveAdmin code</p>
  
  <pre><code>ActiveAdmin.register Promotion do
  
    index do
      actions
      column :name
      column :release_date, sortable: :release_date
    end
  
  end 
  </code></pre>
  
  <p>When I sort by release_date desc, entries with no date bubble to the top. I would prefer if those entries are always at the bottom, regardless of the sort order. Is there any way to specify the compare function when sorting a column?</p>