code: public static TransactionWitness redeemP2WPKH(@Nullable TransactionSignature signature, ECKey pubKey) {
          checkArgument(pubKey.isCompressed(), "only compressed keys allowed");
          TransactionWitness witness = new TransactionWitness(2);
          witness.setPush(0, signature != null ? signature.encodeToBitcoin() : new byte[0]); // signature
          witness.setPush(1, pubKey.getPubKey()); // pubkey
          return witness;
      }
docs: Creates the stack pushes necessary to redeem a P2WPKH output. If given signature is null, an empty push will be
  used as a placeholder.