Fix average attendance annotation returning None for users that doesn't have any RaidResponses for past raids yet.

This occurs because we don't filter users on weather or not they even have RaidResponses for past raids, but only do the filtering on the Avg function. This is correct behaviour, as we want to show *all* users in the attendance matrix. We therefore introduce the 'Coalesce' function to default to an average attendance of 0.

drakul/users/views.py

@@ -1,6 +1,7 @@
 from contextlib import suppress
 
-from django.db.models import Avg, Q
+from django.db.models import Avg, Q, Value
+from django.db.models.functions import Coalesce
 from django.utils import timezone
 from django.views.generic import TemplateView
 
@@ -17,12 +18,15 @@ class AttendanceView(TemplateView):
             .filter(is_active=True) \
             .select_related("main") \
             .annotate(
-                avg_attendance=Avg(
-                    "characters__raid_responses__attendance",
-                    filter=Q(
-                        characters__raid_responses__raid__date__lte=timezone.now(),  # only count past raids
-                        characters__raid_responses__raid__is_optional=False
-                    )
+                avg_attendance=Coalesce(
+                    Avg(
+                        "characters__raid_responses__attendance",
+                        filter=Q(
+                            characters__raid_responses__raid__date__lte=timezone.now(),  # only count past raids
+                            characters__raid_responses__raid__is_optional=False
+                        )
+                    ),
+                    Value(0)  # default to 0 if user doesn't have any responses for past raids yet
                 )
             ).order_by("main__klass", "-avg_attendance", "main__name")