emsm fails with JSON Decoder error

[SilkAndSlug]

This affects at least the guard, worlds, and backups plugins. Everytime I call emsm, I get the below error.

EMSM: Uncaught exception:
 > Exception: JSONDecodeError
 > Module     /usr/lib/python3.7/json/decoder.py
 > Message:   Expecting value: line 1 column 1 (char 0)
 > A full traceback can be found in the log file.

The log ends with:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/emsm/__init__.py", line 41, in run
    app.setup()
  File "/usr/local/lib/python3.7/dist-packages/emsm/core/application.py", line 327, in setup
    self._plugins.init_plugins()
  File "/usr/local/lib/python3.7/dist-packages/emsm/core/plugins.py", line 399, in init_plugins
    plugin = plugin_type(self._app, name)
  File "/usr/local/lib/python3.7/dist-packages/emsm/plugins/guard.py", line 224, in __init__
    self._load_guard_db()
  File "/usr/local/lib/python3.7/dist-packages/emsm/plugins/guard.py", line 324, in     _load_guard_db
    self._guard_db = json.load(file)
  File "/usr/lib/python3.7/json/__init__.py", line 296, in load
    parse_constant=parse_constant, object_pairs_hook=object_pairs_hook, **kw)
  File "/usr/lib/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.7/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

This has been happening for the past few days (since 15:01 BST, 2019-08-06). From that log, I guess I upgraded the JSON package, and it's got a bug. It's a show-stopper for EMSM, hence I've raised it here.

[SilkAndSlug]

I've been poking. The above stacktrace leaves EMSM via guard.py#324, which references plugin_data/guard/errors.json. On my server, that file is empty. Changing it to an empty object:

{
}

fixed the problem. I don't know why the file was empty -- it's not empty in my backups.