Fix backend documentation update

Did overwrite already set props with "".

aleksis/apps/alsijil/schema/documentation.py

@@ -165,9 +165,13 @@ class DocumentationBatchCreateOrUpdateMutation(graphene.Mutation):
             if not info.context.user.has_perm("alsijil.edit_documentation_rule", obj):
                 raise PermissionDenied()
 
-            obj.topic = doc.topic or ""
-            obj.homework = doc.homework or ""
-            obj.group_note = doc.group_note or ""
+            if doc.topic:
+                obj.topic = doc.topic
+            if doc.homework:
+                obj.homework = doc.homework
+            if doc.group_note:
+                obj.group_note = doc.group_note
+
             obj.save()
             return obj