op-node: safer genesis script

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.

op-node/cmd/genesis/cmd.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/ethclient"
+	"github.com/ethereum/go-ethereum/log"
 
 	"github.com/ethereum-optimism/optimism/op-bindings/hardhat"
 	"github.com/ethereum-optimism/optimism/op-chain-ops/genesis"
@@ -108,6 +109,7 @@ var Subcommands = cli.Commands{
 		},
 		Action: func(ctx *cli.Context) error {
 			deployConfig := ctx.String("deploy-config")
+			log.Info("Deploy config", "path", deployConfig)
 			config, err := genesis.NewDeployConfig(deployConfig)
 			if err != nil {
 				return err
@@ -134,7 +136,9 @@ var Subcommands = cli.Commands{
 			}
 
 			var l1StartBlock *types.Block
-			if config.L1StartingBlockTag.BlockHash != nil {
+			if config.L1StartingBlockTag == nil {
+				l1StartBlock, err = client.BlockByNumber(context.Background(), nil)
+			} else if config.L1StartingBlockTag.BlockHash != nil {
 				l1StartBlock, err = client.BlockByHash(context.Background(), *config.L1StartingBlockTag.BlockHash)
 			} else if config.L1StartingBlockTag.BlockNumber != nil {
 				l1StartBlock, err = client.BlockByNumber(context.Background(), big.NewInt(config.L1StartingBlockTag.BlockNumber.Int64()))
@@ -142,6 +146,7 @@ var Subcommands = cli.Commands{
 			if err != nil {
 				return fmt.Errorf("error getting l1 start block: %w", err)
 			}
+			log.Info("Using L1 Start Block", "number", l1StartBlock.Number(), "hash", l1StartBlock.Hash().Hex())
 
 			// Build the developer L2 genesis block
 			l2Genesis, err := genesis.BuildL2Genesis(config, l1StartBlock)