Make files optional when publishing algorithm

[r-marques]

When creating and algorithm the files should be optional (it should exist but it can be empty). This is to allow algorithms that don't provide a file but provide a docker image.

Currently the sdk fails with:

nevermined_sdk_py/nevermined/assets.py", line 197, in create
    assert metadata_copy['main'][
AssertionError: files is required in the metadata main attributes.