Do not update staking list if StakeInfo query failed

Previously, CurrentStakes were getting cleared if stakeInfoReturnValue
== nil, which means that the stake information query has failed.

We still process non-nil but empty stake query result.

node/staking.go

@@ -26,10 +26,12 @@ const (
 	lockPeriodInEpochs = 3 // This should be in sync with contracts/StakeLockContract.sol
 )
 
-// UpdateStakingList updates staking information by querying the staking smart contract.
+// UpdateStakingList updates staking list from the given StakeInfo query result.
 func (node *Node) UpdateStakingList(stakeInfoReturnValue *structs.StakeInfoReturnValue) {
+	if stakeInfoReturnValue == nil {
+		return
+	}
 	node.CurrentStakes = make(map[common.Address]*structs.StakeInfo)
-	if stakeInfoReturnValue != nil {
 	for i, addr := range stakeInfoReturnValue.LockedAddresses {
 		blockNum := stakeInfoReturnValue.BlockNums[i]
 		lockPeriodCount := stakeInfoReturnValue.LockPeriodCounts[i]
@@ -50,7 +52,6 @@ func (node *Node) UpdateStakingList(stakeInfoReturnValue *structs.StakeInfoRetur
 			}
 		}
 	}
-	}
 }
 
 func (node *Node) printStakingList() {