Fix deleting topic, homework or groupNote

aleksis/apps/alsijil/schema.py

@@ -96,11 +96,11 @@ class LessonDocumentationMutation(graphene.Mutation):
             extra_lesson=extra_lesson
         )
 
-        if topic:
+        if topic is not None:
             lesson_documentation.topic = topic
-        if homework:
+        if homework is not None:
             lesson_documentation.homework = homework
-        if group_note:
+        if group_note is not None:
             lesson_documentation.group_note = group_note
 
         lesson_documentation.save()