title: WTForms skips validation when set to optional
body: <p>This field is optional. WTForms does not validate according to a DecimalField if an entry has been made, but is not a decimal?</p>
  <pre><code>field = DecimalField(&quot;field&quot;, validators=[Optional()])
  </code></pre>
  <ul>
  <li>WTForms version is <code>3.0.0</code></li>
  <li>Flask_WTF version is version is <code>1.0.0</code></li>
  </ul>