fix: handle null response from Tempo services API

Signed-off-by: Sandeep20013 <sandeepm20013@gmail.com>

Author: Sandeep20013

tests/model_explainability/guardrails/test_guardrails.py

@@ -389,8 +389,7 @@ def test_guardrails_traces_in_tempo(
 
         @retry(wait_timeout=Timeout.TIMEOUT_1MIN, sleep=5)
         def check_traces():
-            services = requests.get(f"{tempo_traces_service_portforward}/api/services").json().get("data", [])
-
+            services = requests.get(f"{tempo_traces_service_portforward}/api/services").json().get("data") or []
             guardrails_services = [s for s in services if "guardrails" in s]
             if not guardrails_services:
                 return False