KeyError exception when saving a packed workflow

[ghost]

When importing a packed workflow, some output_id use 0 value, which
causes the exception below (when we click on "Save" in workflow editor
to persist modifications):

Traceback (most recent call last):
  File "lib/galaxy/web/framework/decorators.py", line 283, in decorator
    rval = func(self, trans, *args, **kwargs)
  File "lib/galaxy/webapps/galaxy/api/workflows.py", line 552, in update
    **from_dict_kwds
  File "lib/galaxy/managers/workflows.py", line 425, in update_workflow_from_raw_description
    self._sync_stored_workflow(trans, stored_workflow)
  File "lib/galaxy/managers/workflows.py", line 540, in _sync_stored_workflow
    wf_dict = from_galaxy_native(wf_dict, None, json_wrapper=True)
  File "/home/jra001k/snapshot/galaxy/.venv/local/lib/python2.7/site-packages/gxformat2/export.py", line 48, in from_galaxy_native
    source = _to_source(workflow_output, label_map, output_id=step["id"])
  File "/home/jra001k/snapshot/galaxy/.venv/local/lib/python2.7/site-packages/gxformat2/export.py", line 217, in _to_source
    output_id = output_id or has_output_name['id']
KeyError: 'id'

It seems caused by 'or' operator interpreting 0 value as false.

Replacing 'or' with an 'else/if' block fixes the problem.

f68e787f218531dc3d106b81b7d296bf1822d125
(gxformat2 fork)