minor enhancement for test fixtures

llama_stack/providers/tests/agents/fixtures.py

@@ -83,6 +83,6 @@ async def agents_stack(request, inference_model, safety_shield):
             )
             for model in inference_models
         ],
-        shields=[safety_shield],
+        shields=[safety_shield] if safety_shield else [],
     )
     return test_stack

llama_stack/providers/tests/safety/fixtures.py

@@ -47,6 +47,9 @@ def safety_shield(request):
     else:
         params = {}
 
+    if not shield_id:
+        return None
+
     return ShieldInput(
         shield_id=shield_id,
         params=params,