only use file metadata if there are files

pandora/item/models.py

@@ -551,6 +551,7 @@ class Item(models.Model):
         s.risk =   0 #FIXME
         # data from related files
         videos = self.main_videos()
+        if len(videos) > 0:
             s.duration = sum([v.duration for v in videos])
             s.resolution = videos[0].width * videos[0].height
             s.aspectratio = int(1000 * utils.parse_decimal(v.display_aspect_ratio))