title: Hide filter items that produce zero results in django-filter
body: <p>I have an issue with the django-filter application: how to hide the items that will produce zero results. I think that there is a simple method to do this, but idk how.</p>
  
  <p>I'm using the LinkWidget on a ModelChoiceFilter, like this:</p>
  
  <pre><code>provider = django_filters.ModelChoiceFilter(queryset=Provider.objects.all(), 
      widget=django_filters.widgets.LinkWidget) 
  </code></pre>
  
  <p>What I need to do is filter the queryset and select only the Provider that will produce at least one result, and exclude the others.
  There is a way to do that?</p>