title: Default attachment when has_one_attached
body: <p>I have a user model with a <code>has_one_attached :avatar</code>, using <code>ActiveStorage</code>.
  Avatar is optional, but when it isn't present I want to show a default instead. I store all attachments on Amazon S3, and prefer to store the default there as well.</p>
  
  <p>I'd much prefer if I was able to use variant methods on the default avatar as well.  </p>
  
  <pre><code>user.avatar.variant(resize: "100x100")
  </code></pre>
  
  <p>Any suggestions on how to achieve this? Can I create some sort of default attachment that isn't linked to any specific record and use that when the avatar isn't present?</p>