Werk #14284: Notification spooler: Fix producing corrupted spool files

The Notification Spooler could create corrupted spool files which resulted in error messages like this in the var/log/mknotifyd.log log:

2022-07-18 21:49:14,276 [40] [cmk.mknotifyd] Corrupted spoolfile /omd/sites/stable/var/check_mk/notify/spool/ccaae1d5-8b33-403c-bbd8-e1b8796035ca:
Traceback (most recent call last):
  File "/omd/sites/stable/lib/python3/cmk/cee/mknotifyd/spool_handler.py", line 70, in _process_spool_directory
    content: Message = evaluate_file(spoolfile_path)
  File "/omd/sites/stable/lib/python3/cmk/cee/mknotifyd/utils.py", line 50, in evaluate_file
    return ast.literal_eval(f.read())
  File "/omd/sites/stable/lib/python3.9/ast.py", line 62, in literal_eval
    node_or_string = parse(node_or_string, mode='eval')
  File "/omd/sites/stable/lib/python3.9/ast.py", line 50, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 0

SyntaxError: unexpected EOF while parsing
2022-07-18 21:49:14,728 [20] [cmk.mknotifyd] got exit code 2
2022-07-18 21:49:14,728 [40] [cmk.mknotifyd] exception handling notification result 2 for 82bcd0e5-5d28-4162-9f46-84d008e1d73d:
Traceback (most recent call last):
  File "/omd/sites/stable/lib/python3/cmk/cee/mknotifyd/notification_queue.py", line 129, in _handle_result
    content = evaluate_file(path.joinpath(filename))
  File "/omd/sites/stable/lib/python3/cmk/cee/mknotifyd/utils.py", line 50, in evaluate_file
    return ast.literal_eval(f.read())
  File "/omd/sites/stable/lib/python3.9/ast.py", line 62, in literal_eval
    node_or_string = parse(node_or_string, mode='eval')
  File "/omd/sites/stable/lib/python3.9/ast.py", line 50, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 0

SyntaxError: unexpected EOF while parsing

The var/log/notify.log could also show errors in this situation, with messages like this:

2022-07-18 20:40:55,960 [40] [cmk.base.notify] ERROR:
Traceback (most recent call last):
  File "/omd/sites/stable/lib/python3/cmk/base/notify.py", line 1765, in handle_spoolfile
    raw_context = data["context"]
KeyError: 'context'

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