Request with invalid json content type fails with a JSONDecodeError when using the history panel

[israel-tk]

Similar to #1330

If an API request is made with invalid json and the content type application/json, the history panel raises an error and the whole API request fails. django-debug-toolbar 3.1.1.

>>> Client(enforce_csrf_checks=True).post(
            "some-valid-url", data="invalid-json", content_type="application/json",
        )
/usr/local/lib/python3.8/site-packages/django/test/client.py:742: in post
    response = super().post(path, data=data, content_type=content_type, secure=secure, **extra)
/usr/local/lib/python3.8/site-packages/django/test/client.py:404: in post
    return self.generic('POST', path, post_data, content_type,
/usr/local/lib/python3.8/site-packages/django/test/client.py:470: in generic
    return self.request(**r)
/usr/local/lib/python3.8/site-packages/django/test/client.py:710: in request
    self.check_exception(response)
/usr/local/lib/python3.8/site-packages/django/test/client.py:571: in check_exception
    raise exc_value
/usr/local/lib/python3.8/site-packages/django/core/handlers/exception.py:47: in inner
    response = get_response(request)
/usr/local/lib/python3.8/site-packages/debug_toolbar/middleware.py:67: in __call__
    panel.generate_stats(request, response)
/usr/local/lib/python3.8/site-packages/debug_toolbar/panels/history/panel.py:57: in generate_stats
    data = json.loads(
/usr/local/lib/python3.8/json/__init__.py:357: in loads
    return _default_decoder.decode(s)
/usr/local/lib/python3.8/json/decoder.py:337: in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())