Improve admin filter lookups() return type (#992)

Author: adamchainz

django-stubs/contrib/admin/filters.pyi

@@ -23,7 +23,7 @@ class SimpleListFilter(ListFilter):
     parameter_name: str = ...
     lookup_choices: Any = ...
     def value(self) -> Optional[str]: ...
-    def lookups(self, request: HttpRequest, model_admin: ModelAdmin) -> Iterable[Tuple[Any, str]]: ...
+    def lookups(self, request: HttpRequest, model_admin: ModelAdmin) -> Optional[Iterable[Tuple[Any, str]]]: ...
     def choices(self, changelist: Any) -> Iterator[Dict[str, Any]]: ...
 
 class FieldListFilter(ListFilter):