Listing and handling issues with unassigned category

[josephguillaume]

Currently issues with an unassigned category only show up in a renderBoard view for a different category/status. They do not appear in their own category's board view, or in the renderTable view.
Currently new issues that are not immediately assigned values for all defined categories simply never appear in the table view, so this is a major usability problem.

Given the use of an optional sparql query, it appears that the intention was for unassigned/undefined categories to be supported in the table
https://github.com/solid/issue-pane/blob/a213104dfb997a4592f5ff3fe82c2cc6d85a5f8c/issuePane.js#L275-L280

However, table.js does not currently support an undefined value: https://github.com/solid/solid-ui/issues/418

It seems the simplest solution would be to drop support for optional categories (perhaps with some schema validation) and require a default value to be defined. It's not clear what vocab item would be used in that case. State has wf:initialState, but it seems there's nothing equivalent for categories. Maybe there's a need for wf:initialCategory or wf:defaultCategory? Or is there a generic "default subclass" predicate in another vocabulary, given that a wf:issueCategory is just a class with disjoint subclasses?