filter expert: unable to handle empyt-string parameters not_after & not_before

[ghost]

If the not_after or not_before parameter is an empty string instead of null or non-existing, an error like this is raised:

filter-source-ip-exists-expert: Bot initialization failed.
Traceback (most recent call last):
  File "/dev/intelmq/intelmq/lib/bot.py", line 209, in __init__
    self.init()
  File "/dev/intelmq/intelmq/bots/experts/filter/expert.py", line 47, in init
    self.not_after = self.parse_timeattr(self.not_after)
  File "/dev/intelmq/intelmq/bots/experts/filter/expert.py", line 41, in parse_timeattr
    absolute = parser.parse(time_attr)
  File "/usr/lib/python3.8/site-packages/dateutil/parser/_parser.py", line 1368, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
  File "/usr/lib/python3.8/site-packages/dateutil/parser/_parser.py", line 646, in parse
    raise ParserError("String does not contain a date: %s", timestr)
dateutil.parser._parser.ParserError: String does not contain a date: 

The reason is the is not None check, which does not cover empty strings. Empty strings are set by the manager of empty values.