title: Select where not null or empty in mongoid
body: <p>I modified a model so it includes a new field, such as...</p>
  
  <p><code>field :url, :type =&gt; String</code></p>
  
  <p>I use activeadmin, so when I create a new entry <code>@model.url</code> is empty, and in entries created before changing the schema it's nil. How do I select both? I have tried:</p>
  
  <pre><code># Returns nils and strings
  Model.where(:url.ne =&gt; "").count 
  
  # Returns strings and ""
  Model.where(:url.ne =&gt; nil).count
  
  # Returns strings, nils and ""
  Model.where(:url.ne =&gt; ["", nil]).count 
  </code></pre>
  
  <p>Or, if there's a best practice for this kind of scenario please let me know.</p>