serialize_modification_request crashed toward a string 'None' instead of
JSON null for requests whose test_id was nulled by the SET NULL cascade.
Also corrects two comments that still described the debunked ordering
theory instead of the actual SET NULL fix, and fixes a test that
asserted post-cascade state without committing first (the ORM only
picks up out-of-band DB-side SET NULL on already-loaded objects after
expire_on_commit forces a re-read, matching real router behavior).
kitos
fd47db7bea