title: Rails ActiveStorage scope for when file is NOT attached
body: <p>When using ActiveStorage, how do you create a scope for when files are NOT attached.</p>
  
  <p>For example:</p>
  
  <pre><code>class Check &lt; ActiveRecord::Base
    has_one_attached :image
  end
  </code></pre>
  
  <p>I want something like <code>Check.has_no_attached_image</code> to return only records where there is no existing attached image.</p>
  
  <p>Found an <a href="https://stackoverflow.com/a/53199694/3468846">answer</a> for the case where image are attached but not the opposite<br>
  <code>scope :has_attached_image, -&gt; { joins(image_attachment: :blob) }</code></p>