title: Custom value for list_display item on Django admin
body: <p>In a model I have a subtitle field, which is populated with <code>None</code> if no value exists for the field for the given object.</p>
  
  <p>Is there any way to display the value to something custom (like <code>Not Available</code> or <code>Not Applicable</code> than just displaying <code>(None)</code></p>
  
  <p>field</p>
  
  <pre><code>sub_title = models.CharField(max_length=255, null=True, blank=True) 
  </code></pre>
  
  <p>admin</p>
  
  <pre><code> list_display = 'sub_title', 
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/4GG1H.png" rel="noreferrer"><img src="https://i.stack.imgur.com/4GG1H.png" alt="enter image description here"></a></p>
  
  <p>PS: I want <code>None</code> in the database, while a custom value just on admin panel.</p>
  
  <p>thanks</p>