public BlockHeaderBuilder prevRandao(final Bytes32 prevRandao) {
    if (prevRandao != null) {
      this.mixHashOrPrevRandao = prevRandao;
    }
    return this;
  }
A utility class for building block headers.
ethereum/core/src/main/java/org/hyperledger/besu/ethereum/core/BlockHeaderBuilder.java