Handle a non defined configuration in .arkindex.yml

A very simple patch to support non defined configuration (as it is not marked as mandatory here : https://doc.arkindex.org/workers/yaml/#single-worker-configuration). Or maybe I'm wrong and configuration must always be defined ?

Actually it would only fail with the find_model_directory method.

arkindex_worker/worker/base.py

@@ -229,7 +229,7 @@ class BaseWorker(object):
         )
 
         # Retrieve initial configuration from API
-        self.config = worker_version["configuration"].get("configuration")
+        self.config = worker_version["configuration"].get("configuration", {})
         if "user_configuration" in worker_version["configuration"]:
             # Add default values (if set) to user_configuration
             for key, value in worker_version["configuration"][