title: ChoiceField doesn't display an empty label when using a tuple
body: <h2>What I'm trying to do</h2>
  
  <p>I'm going to be keeping data about competitions in my database. I want to be able to search the competitions by certain criteria - competition type in particular.</p>
  
  <h2>About competition types</h2>
  
  <p>Competition types are kept in a tuple. A slightly shortened example:</p>
  
  <pre><code>COMPETITION_TYPE_CHOICES = (
      (1, 'Olympic Games'),
      (2, 'ISU Championships'),
      (3, 'Grand Prix Series'),
  )
  </code></pre>
  
  <p>These are used in the model like so (again - this is a shortened/simplified version of the model):</p>
  
  <pre><code>class Competition(models.Model):
      name = models.CharField(max_length=256)
      type = models.IntegerField(choices=COMPETITION_TYPE_CHOICES) 
  </code></pre>
  
  <h2>The search form</h2>
  
  <p>I don't want the fields to be required in the search form, so the form is defined like this:</p>
  
  <pre><code>class CompetitionSearchForm(forms.Form):
      name = forms.CharField(required=False)
      type = forms.ChoiceField(choices=COMPETITION_TYPE_CHOICES,required=False)
  </code></pre>
  
  <h2>The problem</h2>
  
  <p>I'd like the select widget in ChoiceField to display an empty label, but I don't get one. Any help with this would be much appreciated :)</p>