title: check if matching query does not exist return empty string
body: <p>when i try to check if an object exist return it in excel file filed
  if not return it an empty 'string' 
  but i have error in the <code>if</code> sentence </p>
  
  <pre><code>pkg = Package.objects.get(pk=pk)
  price_date_all = PackagePricesAndDates.objects.filter(package=pkg)
  
  if price_date_all.get(territory=ter.pk).sales:
     list.append(str(price_date_all.get(territory=ter.pk).sales))
  else :
     list.append('')
  </code></pre>