minor fix in saml2

Author: glaubervila

backend/sky_viewer/common/saml2.py

@@ -166,8 +166,10 @@ def get_user_identifier(self, attributes: dict) -> Any:
         try:
             # Lookup value
             user_lookup_value = attributes.get(user_lookup_key, None)
-            if isinstance(user_lookup_value, list) and len(user_lookup_value) > 0:
+
+            if user_lookup_value and isinstance(user_lookup_value, list):
                 user_lookup_value = user_lookup_value[0]
+
                 logger.info(f"User Lookup Value: {user_lookup_value} Type: {type(user_lookup_value)}")
 
             else: