title: Choice - Compare value in input to empty string
body: <p>Using AWS Step Functions with a <a href="https://docs.aws.amazon.com/step-functions/latest/dg/amazon-states-language-choice-state.html" rel="nofollow noreferrer">Choice state</a>, how can one check if a value in the input is equal to an empty string?</p>
  <ul>
  <li><code>IsPresent</code> may be used to check if a particular key exists in the input, but it will return true for an empty string.</li>
  <li><code>StringEquals</code> may be used to compare a string key against a <em>non-empty</em> value</li>
  <li><code>StringMatches</code> may be used to compare a string key against a <em>non-empty</em> wildcard (*) pattern</li>
  </ul>
  <p>Setting <code>StringEmpty</code>'s or <code>StringMatches</code>'s comparison value to empty is invalid <em>in the Workflow Studio UI</em>.</p>
  <h3>Update</h3>
  <p>This is a bug/issue in the current version of the AWS Console Workflow Studio. Comparing to empty is valid, but the UI disallows this configuration via client-side validation.</p>