fix: empty events_iterator assignment

PiperOrigin-RevId: 894188179

Author: google-genai-bot

src/google/adk/sessions/vertex_ai_session_service.py

@@ -176,7 +176,7 @@ async def get_session(
           get_session_response = await api_client.agent_engines.sessions.get(
               name=session_resource_name
           )
-          events_iterator = []
+          events_iterator = None
         else:
           get_session_response, events_iterator = await asyncio.gather(
               api_client.agent_engines.sessions.get(name=session_resource_name),
@@ -210,8 +210,9 @@ async def get_session(
       # to discard events written milliseconds after the session resource was
       # updated. Clock skew between those writes can otherwise drop tool_result
       # events and permanently break the replayed conversation.
-      async for event in events_iterator:
-        session.events.append(_from_api_event(event))
+      if events_iterator is not None:
+        async for event in events_iterator:
+          session.events.append(_from_api_event(event))
 
     if config:
       # Filter events based on num_recent_events.

tests/unittests/sessions/test_vertex_ai_session_service.py

@@ -696,6 +696,20 @@ async def test_get_session_with_many_events(mock_api_client_instance):
   assert len(session.events) == MANY_EVENTS_COUNT
 
 
+@pytest.mark.asyncio
+@pytest.mark.usefixtures('mock_get_api_client')
+async def test_get_session_with_num_recent_events_zero():
+  session_service = mock_vertex_ai_session_service()
+  session = await session_service.get_session(
+      app_name='123',
+      user_id='user',
+      session_id='2',
+      config=GetSessionConfig(num_recent_events=0),
+  )
+  assert session is not None
+  assert len(session.events) == 0
+
+
 @pytest.mark.asyncio
 @pytest.mark.usefixtures('mock_get_api_client')
 async def test_list_sessions():