title: how to implement not-required DateField using Flask-WTF
body: <p>I want a DateField which is optional,
  but I got a "Not a valid date value" error if leave it empty</p>
  
  <p>I add some logs in the source code of wtforms, and found
  formdata.getlist(self.name) returns [u''] for this DateField </p>
  
  <p>The code of my form:</p>
  
  <pre><code>from wtforms import BooleanField, TextField, TextAreaField, PasswordField, validators, HiddenField, DateField, SelectField
  from flask_wtf import Form
  
  class EmployeeForm(Form):
      id = HiddenField('id')
      title = TextField('Title')
      start = DateField('Start Date', format='%m/%d/%Y')
  </code></pre>