item_forms: Allow signals to return None (#4237)

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/control/views/item.py

@@ -1323,6 +1323,8 @@ class ItemUpdateGeneral(ItemDetailMixin, EventPermissionRequiredMixin, MetaDataE
     def plugin_forms(self):
         forms = []
         for rec, resp in item_forms.send(sender=self.request.event, item=self.item, request=self.request):
+            if not resp:
+                continue
             if isinstance(resp, (list, tuple)):
                 forms.extend(resp)
             else: