fix: Add None check for event in remote_a2a_agent.py

PiperOrigin-RevId: 828538350

Author: google-genai-bot

src/google/adk/agents/remote_a2a_agent.py

@@ -372,7 +372,7 @@ def _construct_message_parts_from_session(
       if _is_other_agent_reply(self.name, event):
         event = _present_other_agent_message(event)
 
-      if not event.content or not event.content.parts:
+      if not event or not event.content or not event.content.parts:
         continue
 
       for part in event.content.parts: