Jobs with no input files fail to stage data

[johnbradley]

On the dev server we have a workflow that doesn't stage any input files called word-count.
This workflow fails during staging with the following error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/lando/worker/worker.py", line 170, in run
    self.func(paths, names, self.payload)
  File "/usr/local/lib/python3.5/dist-packages/lando/worker/worker.py", line 39, in stage_files
    single_user_id = self.get_single_dds_user_id(payload.input_files)
  File "/usr/local/lib/python3.5/dist-packages/lando/worker/worker.py", line 56, in get_single_dds_user_id
    raise ValueError("ERROR: No user_id found in input files.")
ValueError: ERROR: No user_id found in input files.