test: satisfy pyright for httpx2 SSE events

Author: Jianke LIN

tests/client/test_streamable_http_response_cleanup.py

@@ -15,7 +15,7 @@ def __init__(self, response: httpx2.Response) -> None:
         self.response = response
 
     async def __aiter__(self):
-        yield ServerSentEvent(event="message", data="", id=None, retry=None)
+        yield ServerSentEvent(event="message", data="", id="", retry=None)
         raise RuntimeError("boom")