Rerun breaks on missing output

[donkirkby]

I haven't tried to reproduce this, but here's what I think happened:

1. Define a container with one or more fixed outputs.
2. Write the code in the container so that one of those outputs doesn't get created. It has to accept the command line argument, but not actually create a file with that path.
3. Run the container, and wait for it to finish.
4. Rerun that run, and wait for the rerun to finish.
5. Click on the rerun to see the inputs and outputs.

Expected behaviour: the missing output won't be in the list, but the rest of the list should appear.

Actual behaviour: an error occurs:

  File "/home/don/git/Kive/kive/container/views.py", line 300, in get_context_data
    data_entries = [dict(**c._asdict()) for c in dataset_comparisons]
  File "/home/don/git/Kive/kive/container/views.py", line 300, in <listcomp>
    data_entries = [dict(**c._asdict()) for c in dataset_comparisons]
AttributeError: 'NoneType' object has no attribute '_asdict'

Analysis

When we thought through the new optional arguments and output directories, I didn't think of the fact that fixed outputs are not always present. If the container doesn't create the file, then there won't be an output dataset. It looks we either have to check for None in the view, or filter None out of the result in the top comparison. The return type annotations aren't quite right in runutils.py. The DatasetComparison is actually optional in a bunch of places.