title: Yup validate is either String or Array of strings
body: <p>I would like to validate that a field is either a string or an array of strings</p>
  
  <p>Here is a <a href="https://codesandbox.io/s/formik-example-jmph7?fontsize=14" rel="noreferrer">minimal failing example</a> which happens to use formik but actually I am doing server side validation using yup.</p>
  
  <pre><code>  {
      email: yup
        .mixed()
        .oneOf([yup.array().of(yup.string()), yup.string()])
        .nullable()
    }
  </code></pre>