fix crash when user ID attribute is missing from reply

Author: Guillaume Rousse

src/satosa/base.py

@@ -151,10 +151,10 @@ def _auth_resp_callback_func(self, context, internal_response):
 
         # If configured construct the user id from attribute values.
         if "user_id_from_attrs" in self.config["INTERNAL_ATTRIBUTES"]:
-            subject_id = [
-                "".join(internal_response.attributes[attr]) for attr in
-                self.config["INTERNAL_ATTRIBUTES"]["user_id_from_attrs"]
-            ]
+            subject_id = []
+            for attr in self.config["INTERNAL_ATTRIBUTES"]["user_id_from_attrs"]:
+                if attr in internal_response.attributes:
+                    subject_id.append("".join(internal_response.attributes[attr]))
             internal_response.subject_id = "".join(subject_id)
 
         if self.response_micro_services: