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 https://github.com/tharsis/ethermint/blob/caa1c5a6c6b7ed8ba4aaf6e0b0848f6be5ba6668/rpc/ethereum/backend/feebackend.go#L29

* Update CHANGELOG.md

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
 		}