private SECPSignature createCommitSeal(final QbftBlock block) {
    final QbftBlock commitBlock = createCommitBlock(block);
    final Hash commitHash = commitBlock.getHash();
    return nodeKey.sign(commitHash);
  }
Construct prepared certificate.

@return the optional PreparedCertificate
consensus/qbft-core/src/main/java/org/hyperledger/besu/consensus/qbft/core/statemachine/QbftRound.java