op-chain-ops: base fee check

Check that the L2 genesis block base fee per gas
is not set to `nil`. If it is, it will cause a segfault
in `op-geth` on startup after the migration.

It is configured with the key: `"l2GenesisBlockBaseFeePerGas"`

op-chain-ops/genesis/db_migration.go

@@ -2,6 +2,7 @@ package genesis
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"math/big"
 
@@ -53,6 +54,10 @@ func MigrateDB(ldb ethdb.Database, config *DeployConfig, l1Block *types.Block, m
 		}, nil
 	}
 
+	if config.L2GenesisBlockBaseFeePerGas == nil {
+		return nil, errors.New("must configure L2 genesis block base fee per gas")
+	}
+
 	underlyingDB := state.NewDatabaseWithConfig(ldb, &trie.Config{
 		Preimages: true,
 	})