More robust handling of metadata command in compute worker

[Didayolo]

The metadata file of a scoring or ingestion program is supposed to contain something like this:

command: python3 scoring.py $argument [...]

However, organizers may provide empty file, or file simply containing a string. In such case, we simply want the command to be set to None instead of having the compute worker crashing.

---

codalab/competitions-v2-compute-worker:latest and codalab/competitions-v2-compute-worker:nvidia updated.