pset: PartiallySignedTransaction::GetUnsignedTx does not work correctly when issuance assets have null values

According to the consensus validation rules, we must supple Null values for issunace/reissuance instead of explicit zero. However, the current code for tries does something else:
https://github.com/ElementsProject/elements/blob/c80e926d6e29d5a9524182767967884091665405/src/psbt.cpp#L126-L132

Fix would be to have
1) two separate if else statements for issunace/re-issuance.
2) force_unblinded is always used. There is no way to do explicit issuances in the current pset workflow.

This calculation causes `walletprocesspsbt` to create signatures that do not validate because the sighash is calculated incorrectly.