Fix cases of calls to `FillPSBT` returning `complete=true` when it's not
the case.
This can happen when some inputs have been signed but the transaction is
subsequently modified, e.g. in the context of PayJoins.
Also fixes a related bug where a finalized hex string is attempted to be
added during `walletprocesspsbt` but a CHECK_NONFATAL causes an abort.
Reported in #30077.
Github-Pull: #30357
Rebased-From: 39cea21ec51b9838669c38fefa14f25c36ae7096
f22b9ca70c