[DAV] Add missing unknown element handling

aleksis/core/util/dav_handler/base.py

@@ -297,8 +297,11 @@ class QueryBase:
 
         for response in self.responses:
             base.current_xml = response.propstats[200]
-            for child in self.children.values():
-                child.process(stage, base, response)
+            for name, child in self.children.items():
+                if child is not None:
+                    child.process(stage, base, response)
+                elif stage == "xml":
+                    response.handle_unknown(name)
 
         base.current_xml = previous_xml