ref: fix type errors in sentry.incidents (#72811)

fixes errors when BaseManager becomes typechecked

<!-- Describe your PR here. -->

src/sentry/incidents/endpoints/serializers/alert_rule.py

@@ -175,9 +175,9 @@ def get_attrs(
                 order_by=F("date_added").desc(),
             )
         )
-        activations = alert_activations_ranked.filter(alert_rule__in=item_list, rank__lte=10)
+        activations_qs = alert_activations_ranked.filter(alert_rule__in=item_list, rank__lte=10)
         activations_by_alert_rule_id = defaultdict(list)
-        for activation in activations:
+        for activation in activations_qs:
             activations_by_alert_rule_id[activation.alert_rule_id].append(activation)
 
         alert_rule_projects = set()

src/sentry/incidents/endpoints/serializers/alert_rule_trigger.py

@@ -50,12 +50,12 @@ def serialize(self, obj, attrs, user, **kwargs):
 class DetailedAlertRuleTriggerSerializer(AlertRuleTriggerSerializer):
     def get_attrs(self, item_list, user, **kwargs):
         triggers = {item.id: item for item in item_list}
-        result: DefaultDict[str, dict[str, list[str]]] = defaultdict(dict)
+        result: dict[str, dict[str, list[str]]] = defaultdict(lambda: defaultdict(list))
         for trigger_id, project_slug in AlertRuleTriggerExclusion.objects.filter(
             alert_rule_trigger__in=item_list
         ).values_list("alert_rule_trigger_id", "query_subscription__project__slug"):
-            exclusions = result[triggers[trigger_id]].setdefault("excludedProjects", [])
-            exclusions.append(project_slug)
+            if project_slug is not None:
+                result[triggers[trigger_id]]["excludedProjects"].append(project_slug)
         return result
 
     def serialize(self, obj, attrs, user, **kwargs):