Various fixups for PR13557

Summary: PSBTInput do nto always serialize its utxo in, so the current behavior is incorrect.

Test Plan: As always, tests will come at the end of PR13557's backport.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4322