private void validateBlockHeader() {
    validateSealableBlockHeader();
    checkState(this.mixHashOrPrevRandao != null, "Missing mixHash or prevRandao");
    checkState(this.nonce.isPresent(), "Missing nonce");
    checkState(this.blockHeaderFunctions != null, "Missing blockHeaderFunctions");
  }
A utility class for building block headers.
ethereum/core/src/main/java/org/hyperledger/besu/ethereum/core/BlockHeaderBuilder.java