Use default value for cancelled value

aleksis/apps/chronos/schema/__init__.py

@@ -195,7 +195,7 @@ class SubstitutionBatchCreateOrUpdateMutation(graphene.Mutation):
                     amends=amended_lesson_event,
                     subject=substitution.subject,
                     comment=substitution.comment or "",
-                    cancelled=substitution.cancelled,
+                    cancelled=substitution.cancelled or False,
                 )
                 if substitution.teachers is not None:
                     obj.teachers.set(Person.objects.filter(pk__in=substitution.teachers))