public static SignatureAlgorithm getInstance() {
    return instance != null
        ? instance
        : SignatureAlgorithmType.DEFAULT_SIGNATURE_ALGORITHM_TYPE.get();
  }
getInstance will always return a valid SignatureAlgorithm and never null. This is necessary in
the unit tests be able to use the factory without having to call setInstance first.

@return SignatureAlgorithm instance
crypto/algorithms/src/main/java/org/hyperledger/besu/crypto/SignatureAlgorithmFactory.java