Compute worker: catch the case where metadata is empty

[Didayolo]

To solve this error:

[2022-11-16 12:04:20,291: INFO/ForkPoolWorker-1] Metadata path is /codabench/tmpvcrq013_/program/metadata
[2022-11-16 12:04:20,291: INFO/ForkPoolWorker-1] Metadata contains:
 None
[2022-11-16 12:04:20,291: INFO/ForkPoolWorker-1] Metadata path is /codabench/tmpvcrq013_/ingestion_program/metadata
[2022-11-16 12:04:20,292: INFO/ForkPoolWorker-1] Metadata contains:
 {'command': 'python $ingestion_program/ingestion.py --input_data_dir=$input --output_dir_ingestion=$output --submission_dir=$submission_program --test_tasks_per_dataset=100 --seed=93 --verbose=True --debug_mode=0 --overwrite_previous_results=True'}
[...]
[2022-11-16 12:04:22,657: ERROR/ForkPoolWorker-1] Task compute_worker_run[47e63c6b-996f-4dde-8652-999fbda64efb] raised unexpected: AttributeError("'NoneType' object has no attribute 'get'")
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/celery/app/trace.py", line 385, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/celery/app/trace.py", line 650, in __protected_call__
    return self.run(*args, **kwargs)
  File "/compute_worker.py", line 84, in run_wrapper
    run.start()
  File "/compute_worker.py", line 675, in start
    loop.run_until_complete(gathered_tasks)
  File "/usr/local/lib/python3.8/asyncio/base_events.py", line 612, in run_until_complete
    return future.result()
  File "/compute_worker.py", line 487, in _run_program_directory
    command = metadata.get("command")
AttributeError: 'NoneType' object has no attribute 'get'