Guard the legacy `image:` migration so it only rewrites shapes the
pre-platform schema actually accepted. A list with a non-dict or
already-platform-tagged entry, or a dict with no recognised image keys
(e.g. `image: ["bad"]`, `image: {foo: 1}`), now returns None from the
migration hook, so normal platform validation surfaces a proper error
instead of the migration silently dropping the input.
Jesse Hills
0b1e3283d5