Remove unreachable defensive code to improve coverage

Removed the else branch handling when url is neither str nor URL,
as the type hint enforces url: URL | str. This was unreachable
defensive code that was causing a coverage gap.

Author: veeceey

httpx/_urls.py

@@ -129,10 +129,8 @@ def __init__(self, url: URL | str = "", **kwargs: typing.Any) -> None:
                             if parsed_url.query
                             else QueryParams()
                         )
-                    elif isinstance(url, URL):
+                    else:  # isinstance(url, URL)
                         existing_params = url.params
-                    else:
-                        existing_params = QueryParams()
 
                     # Merge existing and new params
                     if params: