Add missing test cases for URL constructor edge cases

- Add test for invalid URL type with params parameter (covers line 124)
- Add test for explicit params=None handling (covers line 141)
- Achieves 100% test coverage for httpx/_urls.py

Author: ferstar

tests/models/test_url.py

@@ -467,6 +467,21 @@ class ExternalURLClass:  # representing external URL class
         httpx.URL(ExternalURLClass())  # type: ignore
 
 
+def test_url_invalid_type_with_params():
+    with pytest.raises(TypeError):
+        httpx.URL(123, params={"a": "b"})  # type: ignore
+
+
+def test_url_with_params_none():
+    # Test with existing query parameters
+    url = httpx.URL("https://example.com?existing=param", params=None)
+    assert "existing=param" in str(url)
+
+    # Test without existing query parameters
+    url = httpx.URL("https://example.com", params=None)
+    assert str(url) == "https://example.com"
+
+
 def test_url_with_invalid_component():
     with pytest.raises(TypeError) as exc:
         httpx.URL(scheme="https", host="www.example.com", incorrect="/")