Change default value for user_configuration

Closes #82 (closed)

arkindex_worker/worker/base.py

@@ -62,7 +62,7 @@ class BaseWorker(object):
         logger.info(f"Worker will use {self.work_dir} as working directory")
 
         self.process_information = None
-        self.user_configuration = None
+        self.user_configuration = {}
         self.support_cache = support_cache
         # use_cache will be updated in configure() if the cache is supported and if there
         # is at least one available sqlite database either given or in the parent tasks