Fix empty encoded body sending spurious chunked terminator

src/requests/models.py

@@ -557,7 +557,7 @@ def prepare_body(self, data, files, json=None):
                 (body, content_type) = self._encode_files(files, data)
             else:
                 if data:
-                    body = self._encode_params(data)
+                    body = self._encode_params(data) or None
                     if isinstance(data, basestring) or hasattr(data, "read"):
                         content_type = None
                     else:

tests/test_lowlevel.py

@@ -426,3 +426,33 @@ def response_handler(sock):
     assert isinstance(excinfo.value, requests.exceptions.RequestException)
     assert isinstance(excinfo.value, JSONDecodeError)
     assert r.text not in str(excinfo.value)
+
+
+@pytest.mark.parametrize(
+    "method,include,exclude",
+    (
+        (requests.get, [], [b"Content-Length:", b"Transfer-Encoding:"]),
+        (requests.post, [b"Content-Length: 0\r\n"], [b"Transfer-Encoding:"]),
+    ),
+)
+def test_empty_urlencoded_form_body(method, include, exclude):
+    """Ensure requests with data that encodes to an empty body do not
+    send a spurious chunked transfer terminator (0\\r\\n\\r\\n).
+
+    See https://github.com/psf/requests/pull/6122
+    """
+    close_server = threading.Event()
+    server = Server(echo_response_handler, wait_to_close_event=close_server)
+
+    with server as (host, port):
+        url = f"http://{host}:{port}/"
+        resp = method(url, data=(("a", None),))
+        close_server.set()  # release server block
+
+    assert not resp.content.endswith(b"\r\n0\r\n\r\n")
+
+    for header in include:
+        assert header in resp.content
+
+    for header in exclude:
+        assert header not in resp.content