Server error in Mongo plugin if user has no events

When a user tries to export his calendar through the web interface, a server error is returned if there is no events associated.

In server logs, the following traceback appears

radicale_1  | [2022-09-06 12:15:54 +0000] [1/Thread-171] [ERROR] An exception occurred during GET request on '/bruno boiget/calendar.ics/': max() arg is an empty sequence
radicale_1  | Traceback (most recent call last):
radicale_1  |   File "/usr/lib/python3.8/site-packages/radicale/app/__init__.py", line 105, in __call__
radicale_1  |     status, headers, answers = self._handle_request(environ)
radicale_1  |   File "/usr/lib/python3.8/site-packages/radicale/app/__init__.py", line 276, in _handle_request
radicale_1  |     status, headers, answer = function(
radicale_1  |   File "/usr/lib/python3.8/site-packages/radicale/app/get.py", line 101, in do_GET
radicale_1  |     "Last-Modified": item.last_modified,
radicale_1  |   File "/usr/lib/python3.8/site-packages/radicale_storage_mongo/radicale_mongo.py", line 284, in last_modified
radicale_1  |     last_modified = max(event['updatedAt'] for event in events)
radicale_1  | ValueError: max() arg is an empty sequence