Updates the `op-node` genesis gen script to prevent `nil` pointer
access accidentally. Run the command like so:

```go
go run op-node/cmd/main.go genesis l2 \
  --deploy-config $PWD/packages/contracts-bedrock/deploy-config/mainnet.json \
  --outfile.l2 l2.json \
  --outfile.rollup rollup.json \
  --l1-rpc $ETH_RPC_URL \
  --deployment-dir $PWD/packages/contracts-bedrock/deployments/mainnet
```

When generating the genesis file, it is ok to leave the
`l1StartingBlockTag` value as undefined, this script will select an
appropriate value and then the value will end up inside of the rollup
config under `.genesis.l1.hash` which can then be backported to
the deploy config to make the script deterministic.