Forgejo returns a bare `null` body (not `[]`) for a reactions list when
nothing has reacted yet. issue_reactions/comment_reactions deserialized
straight into forgejo-api's typed Vec<Reaction>, which has no null
tolerance, so issue show / pr show / view failed on every item with zero
reactions - i.e. nearly everything.
Generalize pr_status's existing null_as_empty (same quirk, hit earlier
on combined-status statuses) into a shared helper in verbs/mod.rs, and
fetch reactions via the raw JSON path (Client::get_api_json) with a
NullableVec<T> wrapper instead of the typed client's bare Vec<Reaction>.
Also names the failing request in errors going forward, since
get_api_json's error context includes the URL - closes the gap the
issue itself flagged (the old error said what didn't parse but not
what was fetched).
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