cli: run command failure with no input files

[tiborsimko]

When a workflow does not contain any particular input files, as is my small CVMFS test example:

$ can reana.yaml
workflow:
  type: serial
  resources:
    cvmfs:
    - unpacked.cern.ch
    - cms.cern.ch
  specification:
    steps:
      - environment: 'cern/slc6-base'
        commands:
        - ls -l /cvmfs/cms.cern.ch/ > output2.txt

Then this workflow can be started properly by doing:

$ reana-client create -w cvmfs
$ reana-client start -w cvmfs

but not via run command:

$ reana-client run -w cvmfs
...
  File "/home/simko/.virtualenvs/reana/lib/python3.8/site-packages/reana_client/cli/utils.py", line 59, in wrapper
    return func(*args, **kwargs)
  File "/home/simko/.virtualenvs/reana/lib/python3.8/site-packages/reana_client/cli/utils.py", line 32, in wrapper
    return func(*args, **kwargs)
  File "/home/simko/.virtualenvs/reana/lib/python3.8/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/simko/.virtualenvs/reana/lib/python3.8/site-packages/reana_client/cli/files.py", line 247, in upload_files
    for filename in filenames:
TypeError: 'NoneType' object is not iterable

It would be good to fix the None iteration exception.

[tiborsimko]

Looks good, but I think you haven't made a PR yet?

[tiborsimko]

Looks good, but I think you haven't made a PR yet?

Ah, I see it is a part of the other-titled PR #344 ... OK.