title: How to filter IS NULL in ActiveAdmin?
body: <p>I've a table with an integer column called "map_id", I want to add an activeadmin filter to filter if this column IS NULL or IS NOT NULL.</p>
  
  <p>How could this be implemented ?</p>
  
  <p>I tried the following filter</p>
  
  <pre><code>filter :map_id, :label =&gt; 'Assigned', :as =&gt; :select, :collection =&gt; {:true =&gt; nil, :false =&gt; ''}
  </code></pre>
  
  <p>But, I get the following error message :</p>
  
  <p>undefined method `map_eq' for #</p>