Dataset create via add command produces invalid metadata

[jsam]

Creating a dataset implicitly via add command is producing invalid metadata.

Example:

renku dataset add -c ds1 Dockerfile

and then:

renku dataset

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Traceback (most recent call last):
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/bin/renku", line 11, in <module>
    load_entry_point('renku', 'console_scripts', 'renku')()
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/sam/Work/dev/src/renku-python/renku/cli/exception_handler.py", line 128, in main
    self._handle_github()
  File "/home/sam/Work/dev/src/renku-python/renku/cli/exception_handler.py", line 175, in _handle_github
    getattr(self, '_process_' + value)()
  File "/home/sam/Work/dev/src/renku-python/renku/cli/exception_handler.py", line 119, in main
    result = super().main(*args, **kwargs)
  File "/home/sam/Work/dev/src/renku-python/renku/cli/exception_handler.py", line 90, in main
    return super().main(*args, **kwargs)
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/lib/python3.7/site-packages/click/core.py", line 1114, in invoke
    return Command.invoke(self, ctx)
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/lib/python3.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/sam/Work/dev/src/renku-python/renku/cli/dataset.py", line 383, in dataset
    click.echo(dataset_parent(revision, datadir, format, ctx=ctx))
  File "/home/sam/Work/dev/src/renku-python/renku/core/commands/client.py", line 103, in new_func
    result = ctx.invoke(method, client, *args, **kwargs)
  File "/home/sam/.local/share/virtualenvs/renku-python-jcVNBZ8t/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/sam/Work/dev/src/renku-python/renku/core/commands/dataset.py", line 98, in dataset_parent
    return DATASETS_FORMATS[format](client, datasets)
  File "/home/sam/Work/dev/src/renku-python/renku/core/commands/format/datasets.py", line 41, in tabular
    disable_numparse=[0, 2] if any(datasets) else False
  File "/home/sam/Work/dev/src/renku-python/renku/core/models/tabulate.py", line 48, in tabulate
    ) for c in collection]
  File "/home/sam/Work/dev/src/renku-python/renku/core/models/tabulate.py", line 48, in <listcomp>
    ) for c in collection]
  File "/home/sam/Work/dev/src/renku-python/renku/core/models/datasets.py", line 389, in creators_csv
    return ','.join(creator.short_name for creator in self.creator)
TypeError: 'NoneType' object is not iterable

It seems that the problem is in creating a dataset with no creators.