title: Yup: Validating Array of Strings That Can Be Empty
body: <p>I have the following as a form field type for Formik:</p>
  <pre><code>interface FormFields {
    groups: string[];
  }
  </code></pre>
  <p>I'm trying to pass a Yup schema that will validate the above: the fact that it can be an empty array (must be defined) but can also contain strings.</p>
  <p>The following is not working:</p>
  <pre><code>const schema = Yup.object({
    groups: Yup.array().defined()
  }).defined();
  </code></pre>
  <p>Where am I going wrong?</p>