Remove any NULL characters from displaynames to avoid breaking database txn

Author: Fizzadar

synapse/handlers/user_directory.py

@@ -464,6 +464,10 @@ async def _handle_possible_remote_profile_change(
 
         prev_name = prev_event.content.get("displayname")
         new_name = event.content.get("displayname")
+
+        # Replace any NULL characters in the name as these cannot be stored in the database
+        new_name = new_name.replace("\x00", "\uFFFD")
+
         # If the new name is an unexpected form, do not update the directory.
         if not isinstance(new_name, str):
             new_name = prev_name