Handle "pipelines": [] in config

[JosephMarinier]

When you configure "pipelines": [], the back end currently crashes with an IndexError: list index out of range in postprocessing_known().

I suspect users might try "pipelines": [] if they start with a config with a pipeline and then adapt it to a new dataset, for example, and remove the pipeline.

I see two options:

1. We forbid "pipelines": [], but we handle it clearly in a config validator.
2. We support "pipelines": [] just like if it was "pipelines": null.