txbuilder: fix canSign returning true for missing witness value
txbuilder: fix canSign returning true for missing witness value

bitcoinjs/bitcoinjs-lib,BitGo/bitcoinjs-lib,junderw/bitcoinjs-lib,visvirial/bitcoinjs-lib,UdjinM6/bitcoinjs-lib-dash,bitcoinjs/bitcoinjs-lib,BitGo/bitcoinjs-lib,junderw/bitcoinjs-lib,visvirial/bitcoinjs-lib