Skip BAL validation when not available (#19486)

Temporarily skip BALs when not available. This is due to a missing P2P
spec in devnet-2, so BALs are available only when provided via the CL.
This should be removed once the [BAL P2P
spec](https://eips.ethereum.org/EIPS/eip-8159) has been implemented.

Author: shohamc1

execution/stagedsync/exec3_parallel.go

@@ -200,6 +200,9 @@ func (pe *parallelExecutor) exec(ctx context.Context, execStage *StageState, u U
 						if err != nil {
 							return fmt.Errorf("can't retrieve block %d: for post validation: %w", applyResult.BlockNum, err)
 						}
+						if b == nil {
+							return fmt.Errorf("nil block %d (hash %x)", applyResult.BlockNum, applyResult.BlockHash)
+						}
 
 						lastHeader = b.HeaderNoCopy()
 
@@ -232,17 +235,22 @@ func (pe *parallelExecutor) exec(ctx context.Context, execStage *StageState, u U
 									if err != nil {
 										return fmt.Errorf("block %d: read stored block access list: %w", applyResult.BlockNum, err)
 									}
-									dbBAL, err := types.DecodeBlockAccessListBytes(dbBALBytes)
-									if err != nil {
-										return fmt.Errorf("block %d: read stored block access list: %w", applyResult.BlockNum, err)
-									}
-									if err = dbBAL.Validate(); err != nil {
-										return fmt.Errorf("block %d: db block access list is invalid: %w", applyResult.BlockNum, err)
-									}
+									// BAL data may not be stored for blocks downloaded via backward
+									// block downloader (p2p sync) since it does not carry BAL sidecars.
+									// Remove after eth/71 has been implemented.
+									if dbBALBytes != nil {
+										dbBAL, err := types.DecodeBlockAccessListBytes(dbBALBytes)
+										if err != nil {
+											return fmt.Errorf("block %d: read stored block access list: %w", applyResult.BlockNum, err)
+										}
+										if err = dbBAL.Validate(); err != nil {
+											return fmt.Errorf("block %d: db block access list is invalid: %w", applyResult.BlockNum, err)
+										}
 
-									if headerBALHash != dbBAL.Hash() {
-										log.Info(fmt.Sprintf("bal from block: %s", dbBAL.DebugString()))
-										return fmt.Errorf("block %d: invalid block access list, hash mismatch: got %s expected %s", applyResult.BlockNum, dbBAL.Hash(), headerBALHash)
+										if headerBALHash != dbBAL.Hash() {
+											log.Info(fmt.Sprintf("bal from block: %s", dbBAL.DebugString()))
+											return fmt.Errorf("block %d: invalid block access list, hash mismatch: got %s expected %s", applyResult.BlockNum, dbBAL.Hash(), headerBALHash)
+										}
 									}
 									if headerBALHash != bal.Hash() {
 										log.Info(fmt.Sprintf("computed bal: %s", bal.DebugString()))