eth: fix crash on querying finalized block (#27162)

eth: fix crash on querying nil finalized block

eth/api_backend.go

@@ -134,6 +134,9 @@ func (b *EthAPIBackend) BlockByNumber(ctx context.Context, number rpc.BlockNumbe
 			return nil, errors.New("'finalized' tag not supported on pre-merge network")
 		}
 		header := b.eth.blockchain.CurrentFinalBlock()
+		if header == nil {
+			return nil, errors.New("finalized block not found")
+		}
 		return b.eth.blockchain.GetBlock(header.Hash(), header.Number.Uint64()), nil
 	}
 	if number == rpc.SafeBlockNumber {
@@ -141,6 +144,9 @@ func (b *EthAPIBackend) BlockByNumber(ctx context.Context, number rpc.BlockNumbe
 			return nil, errors.New("'safe' tag not supported on pre-merge network")
 		}
 		header := b.eth.blockchain.CurrentSafeBlock()
+		if header == nil {
+			return nil, errors.New("safe block not found")
+		}
 		return b.eth.blockchain.GetBlock(header.Hash(), header.Number.Uint64()), nil
 	}
 	return b.eth.blockchain.GetBlockByNumber(uint64(number)), nil