Postprocess additional files, or make into things

[towerofnix]

Additional files currently may or may not specify files, and if left unspecified, the value is null. This should be exposed as an empty array instead.

Options include:

• Port additional files to dynamically created things, like contribution objects in Contribution objects! #431
• Post-process additional files to fill in missing values, like commentary entries in Parse and style individual commentary entries! #328

We currently do neither and generally do ?? [] before mapping.

Via Lanolin (#hsmusic-chat).