title: KeyboardDateTimePicker Material UI Not null validation
body: <p>I have implemented KeyboardDateTimePicker. I am not able to validate it for <code>empty</code> or <code>null</code> values? I tried few things for validation, yet its not working.</p>
  <p>Code:</p>
  <pre><code>&lt;MuiPickersUtilsProvider utils={DateFnsUtils}&gt;
      &lt;KeyboardDateTimePicker
      required={true}
      format=&quot;dd/MM/yyyy HH:mm&quot;
      value={this.props.StartDate}
      placeholder=&quot;dd/mm/yyyy hh:mm&quot;
      onChange={(event) =&gt; this.props.handleDateTime(event)}
      /&gt;
  &lt;/MuiPickersUtilsProvider&gt;
  </code></pre>
  <p>I tried placing required, validated=true in form control, yet its not working.</p>
  <p>Did not find any keywords here :
  <a href="https://material-ui-pickers.dev/api/KeyboardDatePicker" rel="nofollow noreferrer">https://material-ui-pickers.dev/api/KeyboardDatePicker</a></p>
  <p>About DateTimePicker:
  <a href="https://material-ui-pickers.dev/demo/datetime-picker" rel="nofollow noreferrer">https://material-ui-pickers.dev/demo/datetime-picker</a></p>
  <p>If anyone has faced similar issue please suggest.</p>