Speedup Range.product_queryset(): exlude Products without parent from join (#4222)

src/oscar/apps/offer/abstract_models.py

                 self.included_categories.values("id")
             )
             _filter |= Q(categories__in=expanded_range_categories)
-            # extend filter for parent categories
-            if Product.objects.filter(
-                parent__categories__in=expanded_range_categories
-            ).exists():
+            # extend filter for parent categories, exclude parent = None
+            if (
+                Product.objects.exclude(parent=None)
+                .filter(parent__categories__in=expanded_range_categories)
+                .exists()
+            ):
                 _filter |= Q(parent__categories__in=expanded_range_categories)
 
         qs = Product.objects.filter(_filter, ~Q(excludes=self))