Checkmk Werk 15709: Fix "Transform spool files" update issue

ID: 15709 Title: Fix "Transform spool files" update issue Component: Core & setup Level: 1 Class: Bug fix Version: 2.2.0p5 Empty files in notification spool directories did break the update procedure in previous 2.2 version, which prevented the update procedure from ensuring a consistent configuration and runtime data. The related error looked like this: C+: -| 18/20 Transform spool files... -| + "Transform spool files" failed -| Traceback (most recent call last): -| File "/omd/sites/semitan/lib/python3/cmk/update_config/main.py", line 223, in _call_ -| action(self._logger, self.update_state.setdefault(action.name)) -| File "/omd/sites/semitan/lib/python3/cmk/update_config/cee/plugins/actions/notifications.py", line 31, in _call_ -| data["context"] = transform_flexible_and_plain_context(data["context"]) -| ~~~~^^^^^^^^^^^ -| KeyError: 'context' C-: