json_params_matcher fails on falsy top-level JSON primitives (0, False, "") #834

Describe the bug

When matching a request body containing a falsy top-level JSON primitive (such as 0, False, or ""), matchers.json_params_matcher fails to match the incoming request because (params or {}) in responses/matchers.py evaluates to {}.

Additional context

In responses/matchers.py line 138:

json_params = (params or {}) if not isinstance(params, list) else params

Because (params or {}) uses boolean truthiness (or), any falsy valid JSON primitive value (0, False, "", 0.0) is replaced with {}.

When a client sends requests.post(url, json=0), request_body is "0" and json_module.loads(request_body) produces 0. The equality comparison json_params == json_body compares {} == 0, which evaluates to False. The mock fails with:
request.body doesn't match: 0 doesn't match {}

Instead, json_params should only default to {} when params is None:

json_params = {} if params is None else params

I checked existing issues and PRs (such as #559 and #814); none currently track falsy top-level scalar JSON primitives.

Version of responses

0.26.3

Steps to Reproduce

import responses
from responses import matchers
import requests

@responses.activate
def test_json_primitive():
    responses.add(
        responses.POST,
        "https://example.com/api",
        body="OK",
        match=[matchers.json_params_matcher(0)],
    )
    # Sending 0 as the JSON body
    resp = requests.post("https://example.com/api", json=0)
    assert resp.status_code == 200

test_json_primitive()

Expected Result

The mock should match the request successfully since the request payload (0) matches the expected matcher parameter (0).

Actual Result

ConnectionError: Connection refused by Responses - the call doesn't match any registered mock.

Request:
- POST https://example.com/api

Available matches:
- POST https://example.com/api request.body doesn't match: 0 doesn't match {}