fix randomness notFirstBlock undefined

[zqhxuyuan]

has error when using manta config:

• we have randomness, but don't have notFirstBlock yet.

  
  root@VM-0-3-ubuntu:~/chopsticks# npx @acala-network/chopsticks@latest --config=configs/manta.yml
  Unable to map [u8; 32] to a lookup index
  [11:12:58.488] INFO (xcm/766006): Manta Parachain RPC listening on port 8000
  /root/.npm/_npx/81ad9c881cb83600/node_modules/@acala-network/chopsticks-core/dist/cjs/blockchain/block-builder.js:116
          head.pushStorageLayer().set((0, _index.compactHex)(meta.query.randomness.notFirstBlock()), _storagelayer.StorageValueKind.Deleted);
                                                                                   ^

TypeError: meta.query.randomness.notFirstBlock is not a function at newHeader (/root/.npm/_npx/81ad9c881cb83600/node_modules/@acala-network/chopsticks-core/dist/cjs/blockchain/block-builder.js:116:86) at async buildBlock (/root/.npm/_npx/81ad9c881cb83600/node_modules/@acala-network/chopsticks-core/dist/cjs/blockchain/block-builder.js:169:20) at async TxPool.buildBlock (/root/.npm/_npx/81ad9c881cb83600/node_modules/@acala-network/chopsticks-core/dist/cjs/blockchain/txpool.js:313:43) at async TxPool.buildBlockIfNeeded (/root/.npm/_npx/81ad9c881cb83600/node_modules/@acala-network/chopsticks-core/dist/cjs/blockchain/txpool.js:295:9)