Description of problem:
dnf system-upgrade currently doesn't work at all. Happened when trying to update from Fedora 29 to Rawhide, and even after having migrated to Rawhide through another method, this message still appears.

Version-Release number of selected component:
dnf-4.0.4-1.fc30

Additional info:
reporter:       libreport-2.9.6
cmdline:        /usr/bin/python3 /usr/bin/dnf system-upgrade
crash_function: raw_decode
exception_type: json.decoder.JSONDecodeError
executable:     /usr/bin/dnf
interpreter:    python3-3.7.1-1.fc30.x86_64
kernel:         4.19.0-1.fc30.x86_64
runlevel:       N 5
type:           Python3
uid:            1000

Truncated backtrace:
decoder.py:355:raw_decode:json.decoder.JSONDecodeError: Expecting value: line 1 column 79 (char 78)

Traceback (most recent call last):
  File "/usr/bin/dnf", line 58, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 179, in user_main
    errcode = main(args)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args, cli_class, option_parser_class)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 95, in _main
    cli.configure(list(map(ucd, args)), option_parser())
  File "/usr/lib/python3.7/site-packages/dnf/cli/cli.py", line 882, in configure
    self._parse_commands(opts, args)
  File "/usr/lib/python3.7/site-packages/dnf/cli/cli.py", line 802, in _parse_commands
    self.command = command_cls(self)
  File "/usr/lib/python3.7/site-packages/dnf-plugins/system_upgrade.py", line 293, in __init__
    self.state = State()
  File "/usr/lib/python3.7/site-packages/dnf-plugins/system_upgrade.py", line 111, in __init__
    self._read()
  File "/usr/lib/python3.7/site-packages/dnf-plugins/system_upgrade.py", line 116, in _read
    self._data = json.load(fp)
  File "/usr/lib64/python3.7/json/__init__.py", line 296, in load
    parse_constant=parse_constant, object_pairs_hook=object_pairs_hook, **kw)
  File "/usr/lib64/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/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 79 (char 78)

Local variables in innermost frame:
self: <json.decoder.JSONDecoder object at 0x7f04f7e2d160>
s: '{"download_status": "downloading", "target_releasever": "rawhide", "exclude": '
idx: 0