fix(fetch): serialise empty array in 'data' as JSON (#2476)

playwright/_impl/_fetch.py

@@ -338,7 +338,7 @@ async def _inner_fetch(
         form_data: Optional[List[NameValue]] = None
         multipart_data: Optional[List[FormField]] = None
         post_data_buffer: Optional[bytes] = None
-        if data:
+        if data is not None:
             if isinstance(data, str):
                 if is_json_content_type(serialized_headers):
                     json_data = data if is_json_parsable(data) else json.dumps(data)

tests/async/test_fetch_global.py

@@ -448,12 +448,18 @@ async def test_should_throw_an_error_when_max_redirects_is_less_than_0(
         assert "'max_redirects' must be greater than or equal to '0'" in str(exc_info)
 
 
-async def test_should_serialize_null_values_in_json(
+async def test_should_serialize_request_data(
     playwright: Playwright, server: Server
 ) -> None:
     request = await playwright.request.new_context()
     server.set_route("/echo", lambda req: (req.write(req.post_body), req.finish()))
-    response = await request.post(server.PREFIX + "/echo", data={"foo": None})
-    assert response.status == 200
-    assert await response.text() == '{"foo": null}'
+    for data, expected in [
+        ({"foo": None}, '{"foo": null}'),
+        ([], "[]"),
+        ({}, "{}"),
+        ("", ""),
+    ]:
+        response = await request.post(server.PREFIX + "/echo", data=data)
+        assert response.status == 200
+        assert await response.text() == expected
     await request.dispose()