issue: defaulting of payload to nested option type inconsistent with revised Candid spec

[crusso]

See test in #5505

Rust PR:
dfinity/candid#679

Spec PR and commit
dfinity/candid#678
dfinity/candid@90af305

(Related slack conversation:

https://dfinity.slack.com/archives/C09GAFNLQ73/p1758527370218129)