op-program: Allow genesis block to have non-zero difficulty.

Normally only post-merge blocks (with difficulty of 0) are allowed, but the genesis block is often configured with non-zero difficulty even for chains that are post-merge from genesis.

op-program/client/l2/engine_backend_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/ethereum-optimism/optimism/op-program/client/l2/engineapi"
 	"github.com/ethereum-optimism/optimism/op-program/client/l2/engineapi/test"
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
 	"github.com/ethereum/go-ethereum/consensus/beacon"
 	"github.com/ethereum/go-ethereum/core"
 	"github.com/ethereum/go-ethereum/core/rawdb"
@@ -149,6 +150,9 @@ func setupOracle(t *testing.T, blockCount int, headBlockNumber int) (*params.Cha
 		L2BlockTime:            2,
 		FundDevAccounts:        true,
 		L2GenesisBlockGasLimit: 30_000_000,
+		// Arbitrary non-zero difficulty in genesis.
+		// This is slightly weird for a chain starting post-merge but it happens so need to make sure it works
+		L2GenesisBlockDifficulty: (*hexutil.Big)(big.NewInt(100)),
 	}
 	l1Genesis, err := genesis.NewL1Genesis(deployConfig)
 	require.NoError(t, err)

op-program/client/l2/engineapi/l2_engine_api.go

@@ -207,7 +207,7 @@ func (ea *L2EngineAPI) ForkchoiceUpdatedV1(ctx context.Context, state *eth.Forkc
 	// Block is known locally, just sanity check that the beacon client does not
 	// attempt to push us back to before the merge.
 	// Note: Differs from op-geth implementation as pre-merge blocks are never supported here
-	if block.Difficulty().BitLen() > 0 {
+	if block.Difficulty().BitLen() > 0 && block.NumberU64() > 0 {
 		return STATUS_INVALID, errors.New("pre-merge blocks not supported")
 	}
 	valid := func(id *engine.PayloadID) *eth.ForkchoiceUpdatedResult {