Add nullable columns without a default

Deploying knowledge bases showed the migration runner doing the wrong thing:

    ALTER TABLE "documents" ADD COLUMN "base_id" VARCHAR(32) DEFAULT ''

`base_id` is nullable and its absent value is NULL, but the runner derived a
default from the column type and backfilled every existing row with the empty
string. Nothing then matched `base_id IS NULL`, so the startup sweep that files
pre-bases documents into a default base would have skipped all of them and the
documents would have stayed invisible.

Nobody lost anything -- the live instance had no documents yet -- but the fault
is general: any nullable column added from here would arrive as "" rather than
NULL, and every "is this set?" check would be wrong about the rows that predate
it. So a default is now emitted only for NOT NULL columns, where SQLite requires
one.

The sweep also accepts "" as meaning unfiled, since a deployment that upgraded
through the previous release has rows holding it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_sharing.py

@@ -272,12 +272,30 @@ def test_an_uploaded_document_always_lands_in_a_base(db, people):
     assert document.base_id is not None
 
 
-def test_documents_predating_bases_are_filed_at_startup(db, people):
+@pytest.mark.parametrize("absent", [None, ""])
+def test_documents_predating_bases_are_filed_at_startup(db, people, absent):
+    """Both spellings of "no base": NULL, and the empty string a release that
+    added the column with a type-derived default left behind.
+
+    The empty string has to be written with foreign keys off, because that is
+    the only way it could ever have got there -- ALTER TABLE ADD COLUMN does not
+    check existing rows, but an UPDATE would.
+    """
+    from sqlalchemy import text
+
     document = documents_service.store_upload(
         db, owner=people["frodo"], payload=b"x", filename="a.txt"
     )
-    document.base_id = None
     db.commit()
+
+    db.execute(text("PRAGMA foreign_keys=OFF"))
+    db.execute(
+        text("UPDATE documents SET base_id = :value WHERE id = :id"),
+        {"value": absent, "id": document.id},
+    )
+    db.commit()
+    db.execute(text("PRAGMA foreign_keys=ON"))
+    db.expire(document)
     assert document not in db.scalars(documents_service.visible(db, people["frodo"]))
 
     assert documents_service.sweep_unfiled(db) == 1