Resolve "Do not send metas=None on create_entity"

Closes #74 (closed) : send an empty dict by default on create_entity().

arkindex_worker/worker/element.py

@@ -31,7 +31,7 @@ class ElementMixin(object):
 
         if missing_slugs:
             raise MissingTypeError(
-                f'Element type(s) {", ".join(missing_slugs)} were not found in the {corpus["name"]} corpus ({corpus["id"]}).'
+                f'Element type(s) {", ".join(sorted(missing_slugs))} were not found in the {corpus["name"]} corpus ({corpus["id"]}).'
             )
 
         return True