Do not look for user_configuration when worker_configuration is None

When there is no user_configuration, the worker execution on Arkindex (does not affect dev mode) gives

File "/usr/local/lib/python3.10/site-packages/arkindex_worker/worker/base.py", line 214, in configure
self.user_configuration = worker_configuration.get("configuration")
AttributeError: 'NoneType' object has no attribute 'get'