Apply suggestions from code review

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: holtskinner

src/a2a/client/client.py

@@ -211,8 +211,8 @@ def __init__(
             middleware = []
         if consumers is None:
             consumers = []
-        self._consumers = consumers or []
-        self._middleware = middleware or []
+        self._consumers = consumers
+        self._middleware = middleware
 
     @abstractmethod
     async def send_message(
@@ -229,7 +229,6 @@ async def send_message(
         pairs, or a `Message`. Client will also send these values to any
         configured `Consumer`s in the client.
         """
-        yield
 
     @abstractmethod
     async def get_task(
@@ -275,7 +274,6 @@ async def resubscribe(
         context: ClientCallContext | None = None,
     ) -> AsyncIterator[Task | Message]:
         """Resubscribes to a task's event stream."""
-        yield
 
     @abstractmethod
     async def get_card(