Gitea may return JSON null for an issue's `labels`, `assignees`, or
`user` fields. `dict.get(key, [])` returns None when the key is present
with a null value (the default is only used for missing keys), so the
list comprehensions raised `'NoneType' object is not iterable` for
otherwise-valid issues. Coalesce with `or []` / `or {}` so empty/null
collections normalize to empty results.
Adds a regression test covering all three null fields.
Latte