txscript: align empty CHECKSIG pubkey validation with Core

The refreshed Bitcoin Core script reference tests cover cases where an
empty CHECKSIG input must still fail pubkey encoding checks before the
interpreter falls back to a generic false result.

Author: Roasbeef

txscript/opcode.go

@@ -1993,6 +1993,10 @@ func opcodeCheckSig(op *opcode, data []byte, vm *Engine) error {
 	// This only applies if tapscript verification isn't active, as this
 	// check is done within the sighash itself.
 	if vm.taprootCtx == nil && len(fullSigBytes) < 1 {
+		if err := vm.checkPubKeyEncoding(pkBytes); err != nil {
+			return err
+		}
+
 		vm.dstack.PushBool(false)
 		return nil
 	}