avoid attribute error on user_config

arkindex_worker/worker/base.py

@@ -210,7 +210,7 @@ class BaseWorker(object):
         self.secrets = {name: self.load_secret(name) for name in required_secrets}
 
         # Load worker run configuration when available
-        worker_configuration = worker_run.get("configuration")
+        worker_configuration = worker_run.get("configuration", {})
         self.user_configuration = worker_configuration.get("configuration")
         if self.user_configuration:
             logger.info("Loaded user configuration from WorkerRun")