title: Django: ORDER BY DESC on FloatField puts null values top?
body: <p>I'm surprised to find that an ORDER BY DESC query on a FloatField in Django returns null fields higher than fields with positive values. </p>
  
  <pre><code># In models.py
  class City(models.Model):
      name = models.CharField(max_length=30)
      country = models.ForeignKey(Country)
      value = models.FloatField(null=True, blank=True)
  # In views.py
      cities = City.objects.filter(country__id=country.id).order_by('-value')
  # In template.html
      {% for city in cities %}{{ city.name }}: {{ city.value }} --{% endfor %}
  </code></pre>
  
  <p>I get back the following:</p>
  
  <pre><code>  London: None -- Paris: None -- Berlin: 84.0 -- Tunis: 15.0 --
  </code></pre>
  
  <p>Instead, I want to return all the entries in descending order, with the None values last (i.e. Berlin, Tunis, London, Paris). Any way to do this?</p>