title: Make a SelectField select nothing by default
body: <p>I have a <code>Form</code> containing a <code>SelectField</code>, something like this:</p>
  
  <pre><code>CHOICES = [
      ('A', 'Apple'),
      ('B', 'Ball'),
      ('C', 'Cat'),
  ]
  
  class MyForm(Form):
      choice = SelectField(choices=CHOICES)
      # ...other fields here... #
  </code></pre>
  
  <p>When I render the form, it shows the SelectField with 'Apple' chosen by default. But I want to have nothing chosen in the beginning, so that the user is forced to make the choice themselves.</p>
  
  <p>Is there a way to render a blank choice, which is displayed by default but is not a valid choice once the form is submitted?</p>