Handle empty list for pulp_href__in filter

Fix bug where empty list for the pulp_href__in filter returns all
results instead of no results.

 The problem is that when using FilterMethod, django-filter just simply
ignores None and empty set and returns the full list of results:

https://github.com/carltongibson/django-filter/blob/e4a70a667a0bf3882fd44b557bc76583d2c65cd1/django_filters/filters.py#L804-L805

There's no way to update the method to prevent this since this logic
happens before the method is called. Instead, this change moves the
method to the filter and adds a None check.

fixes pulp#4437

pulpcore/filters.py

@@ -128,7 +128,12 @@ class IdInFilter(BaseInFilter, filters.UUIDFilter):
 
 
 class HREFInFilter(BaseInFilter, filters.CharFilter):
-    pass
+    def filter(self, qs, value):
+        if value is None:
+            return qs
+
+        pks = [extract_pk(href) for href in value]
+        return qs.filter(pk__in=pks)
 
 
 class PulpTypeFilter(filters.ChoiceFilter):
@@ -271,7 +276,7 @@ class BaseFilterSet(filterset.FilterSet):
 
     help_text = {}
     pulp_id__in = IdInFilter(field_name="pk", lookup_expr="in")
-    pulp_href__in = HREFInFilter(field_name="pk", method="filter_pulp_href")
+    pulp_href__in = HREFInFilter(field_name="pk")
     q = ExpressionFilter()
 
     FILTER_DEFAULTS = {
@@ -307,11 +312,6 @@ class BaseFilterSet(filterset.FilterSet):
         "ne": _("not equal to"),
     }
 
-    def filter_pulp_href(self, queryset, name, value):
-        # Convert each href to a pk
-        pks = [extract_pk(href) for href in value]
-        return queryset.filter(pk__in=pks)
-
     @classmethod
     def get_filters(cls):
         filters = super().get_filters()