Integration lane caught it (run 2984): the backfill reported 0 rows to fill
and left `data` unset. `IS NULL` was the whole predicate, but a JSONB column
has two empty states. SQLAlchemy's JSON types default to
`none_as_null=False`, so assigning Python `None` persists the JSON encoding
of null — `IS NULL` walks straight past it.
Migration 0070 left genuine SQL NULLs, so the product path was right; the
test was constructing the wrong shape with `data=None`. Fixed both ways,
because both states mean "no usable mirror":
- predicate is now `data IS NULL OR jsonb_typeof(data) = 'null'`;
- the legacy-row test OMITS `data` (a real SQL NULL, 0070's actual shape),
and a second test covers the JSON-null shape and asserts the premise with
`jsonb_typeof` rather than assuming it.
Refs #2083.
bvandeusen
083944f0fd