Jobs with no input files fail to stage data

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.
```