Update serializers to validate mod settings correctly

pydis_site/apps/api/serializers.py

@@ -709,7 +709,8 @@ def to_representation(self, instance: User) -> dict:
         if hasattr(instance, 'mod_settings') and instance.mod_settings is not None:
             ret['mod_settings'] = UserModSettingsSerializer(instance.mod_settings).data
         else:
-            del ret['mod_settings']
+            if 'mod_settings' in ret:
+                del ret['mod_settings']
 
         return ret
 
@@ -752,11 +753,21 @@ def get_alts(self, user: User) -> list[dict]:
 class UserModSettingsSerializer(ModelSerializer):
     """A class to serialize the moderator settings for a user."""
 
+    def validate(self, data: dict) -> dict:
+        """Validate the moderator settings contain the necessary fields."""
+        if data.get("pings_schedule_start") and not data.get("pings_schedule_end"):
+            raise ValidationError("missing pings_schedule_end")
+
+        if data.get("pings_schedule_end") and not data.get("pings_schedule_start"):
+            raise ValidationError("missing pings_schedule_start")
+
+        return data
+
     class Meta:
         """Meta settings for the user moderator settings serializer."""
 
         model = UserModSettings
-        fields = ('pings_disabled_until', 'pings_schedule_start', 'pings_schedule_end')
+        fields = ('moderator', 'pings_disabled_until', 'pings_schedule_start', 'pings_schedule_end')
 
 class NominationEntrySerializer(ModelSerializer):
     """A class providing (de-)serialization of `NominationEntry` instances."""