title: Add non existing element to array in DynamoDB
body: <p>Im trying to update items attribute that is a list of strings.
  Can I update (append) the attribute only if it not exist .
  Kind of list_append &amp; if_not_exists.</p>
  
  <blockquote>
    <p>var params =  {    ...</p>
  
  <pre><code>UpdateExpression: 
  </code></pre>
    
    <p>'SET friends = list_append(if_not_exists(friends, :empty_list), :new_friend)',</p>
  
  <pre><code>ExpressionAttributeValues:{
      ":new_friend": [{"S":"Bobo"}],
      ":empty_list" :[]
   } };
  </code></pre>
  </blockquote>
  
  <p>this is not working, is there a way?
  so if Bobo still not in my "friends" list it will append it</p>