fix(evm): TxIndex is wrong when trace is the first tx of a block (#1187)

* Fix TxIndex wrongly when trace first Tx of a block

If `Predecessors` is empty, `txConfig.TxIndex` originally = 0 and when it reachs line 401 `txConfig.TxIndex++`, it will becomes 1. It should be 0

* apply suggestion and add change-log

* fix comment

Co-authored-by: Federico Kunze Küllmer <31522760+fedekunze@users.noreply.github.com>

CHANGELOG.md

@@ -53,6 +53,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Bug Fixes
 
+* (evm) [\#1187](https://github.com/evmos/ethermint/pull/1187) Fix `TxIndex` value (expected 0, actual 1) when trace the first tx of a block via `debug_traceTransaction` api
 * (rpc) [\#1190](https://github.com/evmos/ethermint/issues/1190) Fix `UnmarshalJSON` panig of breaking EVM and fee market `Params`.
 * (rpc) [#1179](https://github.com/evmos/ethermint/pull/1179) Fix gas used in traceTransaction response.
 

x/evm/keeper/grpc_query.go

@@ -398,7 +398,10 @@ func (k Keeper) TraceTx(c context.Context, req *types.QueryTraceTxRequest) (*typ
 
 	tx := req.Msg.AsTransaction()
 	txConfig.TxHash = tx.Hash()
-	txConfig.TxIndex++
+	if len(req.Predecessors) > 0 {
+		txConfig.TxIndex++
+	}
+
 	result, _, err := k.traceTx(ctx, cfg, txConfig, signer, tx, req.TraceConfig, false)
 	if err != nil {
 		// error will be returned with detail status from traceTx