Catch JSONDecodeError in wiki.py

https://sentry.fsmpi.rwth-aachen.de/protokollsystem/protokollsystem/issues/7/

JSONDecodeError: Expecting value: line 1 column 1 (char 0)
(5 additional frame(s) were not displayed)
...
  File "program/wiki.py", line 120, in do_action
    return req.json()
  File "requests/models.py", line 892, in json
    return complexjson.loads(self.text, **kwargs)
  File "json/__init__.py", line 319, in loads
    return _default_decoder.decode(s)
  File "json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "json/decoder.py", line 357, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None

JSONDecodeError: Expecting value: line 1 column 1 (char 0)