[Gnosis] Don't call ExecuteSystemWithdrawals before Shanghai (#7160)

This is a patch to PR #6940. Withdrawal contract should not be called
for pre-Shanghai block. The issue was found on
gnosis_withdrawals_devnet_2 (PR #7150), causing
```
[WARN] [03-22|10:44:51.574] [7/15 Execution] Execution failed        block=51035 hash=0xa8fb9e58eb734b7ce4e2e6260ad20e07a16039325f9924cc18ea61fa2eb5ee90 err="execution reverted"
```

Author: yperbasis

consensus/aura/aura.go

@@ -1434,6 +1434,9 @@ func (c *AuRa) ExecuteSystemWithdrawals(withdrawals []*types.Withdrawal, syscall
 	}
 
 	_, err = syscall(*c.cfg.WithdrawalContractAddress, packed)
+	if err != nil {
+		log.Warn("ExecuteSystemWithdrawals", "err", err)
+	}
 	return err
 }
 

consensus/serenity/serenity.go

@@ -134,8 +134,10 @@ func (s *Serenity) Finalize(config *chain.Config, header *types.Header, state *s
 		if err := auraEngine.ApplyRewards(header, state, syscall); err != nil {
 			return nil, nil, err
 		}
-		if err := auraEngine.ExecuteSystemWithdrawals(withdrawals, syscall); err != nil {
-			return nil, nil, err
+		if withdrawals != nil {
+			if err := auraEngine.ExecuteSystemWithdrawals(withdrawals, syscall); err != nil {
+				return nil, nil, err
+			}
 		}
 	} else {
 		for _, w := range withdrawals {