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.

requested review from @schneider-y

assigned to @vrigal

added P4 Quick Win labels

approved this merge request

merged