Quick fix for eth_feeHistory when reward is nil (#975)

* Quick fix for eth_feeHistory when reward is nil

`RPC method eth_feeHistory crashed: runtime error: invalid memory address or nil pointer dereference`
from caa1c5a6c6/rpc/ethereum/backend/feebackend.go (L29)

* Update CHANGELOG.md

CHANGELOG.md

@@ -40,6 +40,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Bug Fixes
 
+* (rpc) [\#529](https://github.com/tharsis/ethermint/pull/975) Fix unexpected `nil` values for `reward`, returned by `EffectiveGasTipValue(blockBaseFee)` in the `eth_feeHistory` RPC method.
 * (evm) [\#529](https://github.com/tharsis/ethermint/issues/529) Add support return value on trace tx response.
 * (rpc) [#970] (https://github.com/tharsis/ethermint/pull/970) Fix unexpected nil reward values on `eth_feeHistory` response
 

rpc/ethereum/backend/feebackend.go

@@ -87,6 +87,9 @@ func (e *EVMBackend) processBlock(
 			}
 			tx := ethMsg.AsTransaction()
 			reward := tx.EffectiveGasTipValue(blockBaseFee)
+			if reward == nil {
+				reward = big.NewInt(0)
+			}
 			sorter[i] = txGasAndReward{gasUsed: txGasUsed, reward: reward}
 			break
 		}