fix: empty log topics shouldn't be encoded as nil (#840)

* Problem: empty topics shouldn't be encoded as nil

Closes: #839

Solution:
- encode it as empty array

* fix unit tests

* changelog

CHANGELOG.md

@@ -37,6 +37,10 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ## Unreleased
 
+### State Machine Breaking
+
+- (evm) [tharsis#840](https://github.com/tharsis/ethermint/pull/840) Store empty topics as empty array rather than nil.
+
 ### Improvements
 
 * (evm) [tharsis#826](https://github.com/tharsis/ethermint/issues/826) Improve allocation of bytes of `tx.To` address.

x/evm/keeper/statedb_test.go

@@ -592,6 +592,7 @@ func (suite *KeeperTestSuite) TestAddLog() {
 			&ethtypes.Log{
 				Address: addr,
 				TxHash:  txHash,
+				Topics:  make([]common.Hash, 0),
 			},
 			func() {},
 		},
@@ -606,6 +607,7 @@ func (suite *KeeperTestSuite) TestAddLog() {
 				TxHash:  txHash2,
 				TxIndex: 1,
 				Index:   1,
+				Topics:  make([]common.Hash, 0),
 			},
 			func() {
 				suite.app.EvmKeeper.SetTxHashTransient(txHash)
@@ -624,6 +626,7 @@ func (suite *KeeperTestSuite) TestAddLog() {
 			&ethtypes.Log{
 				Address: addr,
 				TxHash:  txHash3,
+				Topics:  make([]common.Hash, 0),
 			},
 			func() {},
 		},
@@ -638,6 +641,7 @@ func (suite *KeeperTestSuite) TestAddLog() {
 				TxHash:  txHash4,
 				TxIndex: 1,
 				Index:   1,
+				Topics:  make([]common.Hash, 0),
 			},
 			func() {
 				suite.app.EvmKeeper.SetTxHashTransient(txHash)

x/evm/types/logs.go

@@ -70,9 +70,9 @@ func (log *Log) Validate() error {
 
 // ToEthereum returns the Ethereum type Log from a Ethermint proto compatible Log.
 func (log *Log) ToEthereum() *ethtypes.Log {
-	var topics []common.Hash // nolint: prealloc
+	topics := make([]common.Hash, len(log.Topics))
-	for i := range log.Topics {
+	for i, topic := range log.Topics {
-		topics = append(topics, common.HexToHash(log.Topics[i]))
+		topics[i] = common.HexToHash(topic)
 	}
 
 	return &ethtypes.Log{
@@ -108,9 +108,9 @@ func LogsToEthereum(logs []*Log) []*ethtypes.Log {
 
 // NewLogFromEth creates a new Log instance from a Ethereum type Log.
 func NewLogFromEth(log *ethtypes.Log) *Log {
-	var topics []string // nolint: prealloc
+	topics := make([]string, len(log.Topics))
-	for _, topic := range log.Topics {
+	for i, topic := range log.Topics {
-		topics = append(topics, topic.String())
+		topics[i] = topic.String()
 	}
 
 	return &Log{