Support workers without revisions

Arkindex 1.5.2 will make WorkerVersion.revision and Worker.repository nullable, in the case where a worker is created manually in the frontend. It is possible to specify a Docker image reference when creating a worker version in the frontend, so there can be a WorkerVersion without a revision but that has a Docker image.

Adding a worker with this method then running it causes the worker to crash:

Traceback (most recent call last):
  File "/usr/local/bin/worker-ufcn", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.10/dist-packages/worker_ufcn/worker.py", line 467, in main
    UFCNDetector(description="U-FCN", support_cache=True).run()
  File "/usr/local/lib/python3.10/dist-packages/arkindex_worker/worker/__init__.py", line 170, in run
    self.configure()
  File "/usr/local/lib/python3.10/dist-packages/worker_ufcn/worker.py", line 213, in configure
    super().configure()
  File "/usr/local/lib/python3.10/dist-packages/arkindex_worker/worker/__init__.py", line 160, in configure
    super().configure()
  File "/usr/local/lib/python3.10/dist-packages/arkindex_worker/worker/base.py", line 251, in configure
    f"Loaded worker {self.worker_details['name']} revision {worker_version['revision']['hash'][0:7]} from API"
TypeError: 'NoneType' object is not subscriptable