rpcwallet: disallow empty UTXO pk script [skip ci]
rpcwallet: disallow empty UTXO pk script [skip ci]

If we're signing for an UTXO that isn't known to the wallet, then the
UTXO's pk script _must_ be set in the sign descriptor. Otherwise we run
into a generic PSBT serialization error when running in a remote signing
setup.

aakselrod/lnd,aakselrod/lnd