transaction: Properly handle nil bytes in binary transactions

If returned binaries are nil, we should pass them as nil and not as an
empty bytes array.

Author: 3v1n0

transaction.go

@@ -188,6 +188,8 @@ func cbPAMConv(s C.int, msg *C.char, c C.uintptr_t) (*C.char, C.int) {
 			bytes, err := cb.RespondPAMBinary(BinaryPointer(msg))
 			if err != nil {
 				return nil, ConvAgain.toC()
+			} else if bytes == nil {
+				return nil, Success.toC()
 			}
 			return (*C.char)(C.CBytes(bytes)), Success.toC()
 		} else {