Send empty dict as metas instead of None

arkindex_worker/worker/entity.py

@@ -21,7 +21,7 @@ class EntityType(Enum):
 
 class EntityMixin(object):
     def create_entity(
-        self, element, name, type, corpus=None, metas=None, validated=None
+        self, element, name, type, corpus=None, metas=dict(), validated=None
     ):
         """
         Create an entity on the given corpus through API