Return 0 L1 fees (not nil)

Returning nil works inside geth, but optimism tests expect the
l1CostFunc to return non-nil values. Considering our l1CostFunc is the
only one retuning nil values, it seems more pragmatic to change that
rather than adapting external code to deal correctly with nil values.

core/types/rollup_cost.go

@@ -125,7 +125,7 @@ func NewL1CostFunc(config *params.ChainConfig, statedb StateGetter) L1CostFunc {
 	var cachedFunc l1CostFunc
 	selectFunc := func(blockTime uint64) l1CostFunc {
 		if config.IsCel2(blockTime) {
-			return func(rcd RollupCostData) (fee, gasUsed *big.Int) { return nil, nil }
+			return func(rcd RollupCostData) (fee, gasUsed *big.Int) { return new(big.Int), new(big.Int) }
 		}
 		if !config.IsOptimismEcotone(blockTime) {
 			return newL1CostFuncBedrock(config, statedb, blockTime)