fix: return error instead of nil in ethabi EncodeToEth methods (#4784)

Six ethabi files incorrectly returned nil instead of err when
Outputs.Pack() failed, silently swallowing the error. This fix
ensures the actual error is propagated to callers.

Files fixed:
- action/protocol/rewarding/ethabi/availablebalance.go
- action/protocol/rewarding/ethabi/totalbalance.go
- action/protocol/rewarding/ethabi/unclaimedbalance.go
- action/protocol/staking/ethabi/common/candidatebyname.go
- action/protocol/staking/ethabi/common/stake_totalstakingamount.go
- action/protocol/staking/ethabi/common/types.go

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: envestcc

action/protocol/rewarding/ethabi/availablebalance.go

@@ -59,7 +59,7 @@ func (r *AvailableBalanceStateContext) EncodeToEth(resp *iotexapi.ReadStateRespo
 
 	data, err := _availableBalanceMethod.Outputs.Pack(total)
 	if err != nil {
-		return "", nil
+		return "", err
 	}
 
 	return hex.EncodeToString(data), nil

action/protocol/rewarding/ethabi/totalbalance.go

@@ -59,7 +59,7 @@ func (r *TotalBalanceStateContext) EncodeToEth(resp *iotexapi.ReadStateResponse)
 
 	data, err := _totalBalanceMethod.Outputs.Pack(total)
 	if err != nil {
-		return "", nil
+		return "", err
 	}
 
 	return hex.EncodeToString(data), nil

action/protocol/rewarding/ethabi/unclaimedbalance.go

@@ -81,7 +81,7 @@ func (r *UnclaimedBalanceStateContext) EncodeToEth(resp *iotexapi.ReadStateRespo
 
 	data, err := _unclaimedBalanceMethod.Outputs.Pack(total)
 	if err != nil {
-		return "", nil
+		return "", err
 	}
 
 	return hex.EncodeToString(data), nil

action/protocol/staking/ethabi/common/candidatebyname.go

@@ -79,7 +79,7 @@ func (r *CandidateByNameStateContext) EncodeToEth(resp *iotexapi.ReadStateRespon
 
 	data, err := r.Method.Outputs.Pack(cand)
 	if err != nil {
-		return "", nil
+		return "", err
 	}
 	return hex.EncodeToString(data), nil
 }

action/protocol/staking/ethabi/common/stake_totalstakingamount.go

@@ -59,7 +59,7 @@ func (r *TotalStakingAmountStateContext) EncodeToEth(resp *iotexapi.ReadStateRes
 
 	data, err := r.Method.Outputs.Pack(total)
 	if err != nil {
-		return "", nil
+		return "", err
 	}
 	return hex.EncodeToString(data), nil
 }

action/protocol/staking/ethabi/common/types.go

@@ -100,7 +100,7 @@ func EncodeVoteBucketListToEth(outputs abi.Arguments, buckets *iotextypes.VoteBu
 
 	data, err := outputs.Pack(args)
 	if err != nil {
-		return "", nil
+		return "", err
 	}
 	return hex.EncodeToString(data), nil
 }