title: Return null if key does not exist in VTL / API Gateway Template
body: <p>The following template returns <code>''</code> if the Authorization header is <strong>not</strong> present. How can i get <code>null</code> instead? So return <code>null</code> if key is not present...</p>
  
  <pre><code>{
    "headers" : { 
       "authorization" : "$input.params().header.get('Authorization')" 
    }
  }
  </code></pre>