title: isValid stays false after form validates
body: <p>I have a custom validate function but even when it returns no errors the form is still invalid.</p>
  
  <p>I am passing the following properties to Formik:</p>
  
  <pre><code>validate={({ import_files }) =&gt; {
      return [...import_files.values()].length === 0
          ? { import_files: 'Please choose a file to import' }
          : {};
  }}
  onSubmit={onSubmit}
  initialValues={{ import_files: new Map([]) }}
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/l53Ou.jpg" rel="nofollow noreferrer">Here is a console log of the formik object after validation</a></p>
  
  <p>As you can see the field is touched, it has no errors but <code>isValid</code> is still <code>false</code>.
  Why is the form still invalid?</p>