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

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:

-| 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'

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