fix: ensure safe access to user attributes during sorting in API utility

Author: BryanttV

openedx_authz/rest_api/utils.py

@@ -76,5 +76,5 @@ def sort_users(
     if order not in SortOrder.values():
         raise ValueError(f"Invalid order: '{order}'. Must be one of {SortOrder.values()}")
 
-    sorted_users = sorted(users, key=lambda user: (user[sort_by] or "").lower(), reverse=order == SortOrder.DESC)
+    sorted_users = sorted(users, key=lambda user: (user.get(sort_by) or "").lower(), reverse=order == SortOrder.DESC)
     return sorted_users