datasette --default-deny --root with no config file previously 500'd on
the instance and database index pages: rendering them computes is_private
(include_is_private=True), which references the anon_rules CTE, but that
CTE was only defined when anonymous permission rules existed.
This was fixed by the empty-anon_rules fallback added in 4b5fac9c; this
commit adds a regression test that fails without that fallback (SQLite
"no such table: anon_rules" -> 500).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>